History

Communicable Diseases, Table of Content s

CHAPTER II

INFLAMMATORY DISEASES OF THE RESPIRATORY TRACT (BRONCHITIS, INFLUENZA, BRONCHOPNEUMONIA, LOBAR PNEUMONIA)

In approaching the consideration of the serious and fatal inflammations of the respiratory tract which formed by far the most important factor in the sickness and death records of the Army during the World War, it is first necessary to take a general view of the subject in the attempt to determine, if possible, the causes that led to the large morbidity and mortality from respiratory diseases in general, rather than to limit ourselves to the consideration of each form of disease separately.

The mortality from respiratory diseases during the World War was due almost entirely to pneumonia, primary or secondary.¹ In any set of communities the size of the mobilization camps of the Army during the war, pneumonia is to be expected to some extent. The usual type of pneumonia occurring among young male adults in civil life is of course primary lobar pneumonia, running a fairly definite course and, usually, recognized easily both clinically and post mortem. That such cases occurred among the troops is beyond question. The proportion of such cases, however, is impossible to determine. McCallum expressed the opinion, after studying the pneumonias at Camp Travis, Tex., in the late winter of 1917-18, that they were relatively few in number and distinguished mainly by their mildness as compared to those seen in civil communities.²However, it was early recognized clinically that in the larger number of cases observed in the camps the pneumonia was of an atypical nature. The onset tended to be slower than that of the lobar pneumonia of civil life; the course more prolonged. Crisis was relatively rare; physical signs were slow of development and of patchy distribution and scattered in several lobes. These facts led careful observers to consider a large proportion of the cases as bronchopneumonia rather than as the usual lobar type. The results of post-mortem study of fatal cases lent confirmation to this distinction: The typical croupous consolidation of lobar pneumonia was relatively rare, patchy consolidation of a suppurative character more frequent. Even when the consolidation involved nearly or quite an entire lobe, careful study often showed evidence of the formation of such lobar consolidation by the confluence of smaller areas, lobular in origin.

Inasmuch as bronchopneumonia is almost invariably a complicating or secondary, rather than a primary infection, and its incidence in men of military age, generally speaking, is very low as compared to that of the lobar type, attention was at once focused on the coincident epidemic of measles as the probable primary cause of the pneumonias. That this disease was indeed a large factor in the causation of the pneumonias of the early days of the mobilization camps of the World War is shown in the consideration of that disease.

^aUnless otherwise stated, all figures for the World War period are derived from sick and wounded reports sent to the Surgeon General.-Ed.

However, in many, if not in most camps, the peak of thepneumonia incidence did not coincide with that of the measles and in the lightof subsequent events attention is directed to the possible occurrence in thecamps at this time of another disease also complicated by fatal pneumonia,namely, influenza.

It will be shown that influenza, at least in its so-called endemic form, was a considerable factor in the sick rates of the Army for some years before the World War. It has always been held responsible for a small but varying mortality in the civil population according to the reports of the Census Bureau. It is the consensus of opinion of those who have investigated the subject that minor but distinct epidemic waves of this disease have occurred every few years, in each instance accompanied by an increase in the pneumonia mortality. Cases of influenza were reported from the camps from the earliest days of the mobilization; doubtless many more cases were not recognized owing to the mildness of the type prevailing during the fall of 1917. It is impossible, therefore, to estimate the number of influenza cases that occurred among the troops during these early months. But that the disease was present will be shown in a manner that will leave very little room for doubt, and its fluctuations from month to month, as shown by its effect on the number of admissions for the total respiratory diseases and by its effect on the amount and character of the prevalent pneumonia, can be shown with some definiteness.

Unfortunately for the exactness of our records in this class of diseases the clinical characteristics of mild influenza are such as to lead to its ready confusion with several of the milder so-called common respiratory diseases. Of these, bronchitis, tonsillitis, and pharyngitis are the leading diseases with which many of the earlier cases of influenza were confused. When the outbreak was at its height the uniformity of symptoms presented by large numbers of cases made confusion almost impossible and at the time of an epidemic wave in the majority of instances the cases were correctly diagnosed. However, in certain camps there were pathological purists who refused to give sanction to the diagnosis of influenza unless it was possible to demonstrate the presence of the bacillus of Pfeiffer. This attitude was evidently extreme, in view of the doubt cast in recent years on the specificity of the r?le of this organism in influenza; but the fact remains that in all of the epidemic waves to be described, even in the generally recognized fall outbreak of 1918, there was not only an increase in the number of cases diagnosed as influenza but also a corresponding increase in the "other respiratory diseases." One camp reported a preponderant number of influenza cases, another simultaneously suffering from the same epidemic wave reported few influenza cases, but a great increase in the common respiratory diseases. Even in the 1918 fall wave, three camps-Fremont, Calif.; Gordon, Ga.; and Wheeler, Ga.³-apparently insisted on a bacteriological diagnosis, which was not forthcoming, and reported their epidemic cases as "other respiratory diseases." These two factors then, the impossibility of making an exact clinical diagnosis of influenza in the absence of the great outbreak, and the insistence by some on the bacteriological diagnosis even in the presence of undoubted waves of the disease, make it impracticable to base conclusions as to the varying incidence of influenza in the Army camps on the reported cases of that disease alone. In studying the varying incidence of influenza, therefore,

it becomes necessary to use not only the figures for thatdisease as reported, but also those for certain other acute respiratoryinfections. In using this combined figure we are undoubtedly including a certainnumber of noninfluenzal cases. In view of the number of cases involved, however,and of the more or less constant incidence of these diseases as usuallyobserved, it is believed that the use of this figure will give the most reliablecomparative index of the month-to-month incidence of influenza that it ispossible to obtain.

The study of the relations between the incidence of thecommon respiratory diseases and of the pneumonias, therefore, should serve tothrow light both on the causation of the high pneumonia incidence and mortalityas well as on the character of the responsible primary infection. For thisreason, it seems impossible to consider separately the epidemiology of thepneumonias and of influenza.

It should be understood in studying the various chartspresented that the system of recording admissions for disease in use in the Armyduring the World War referred each case back to date of admission. Thus, if aman was admitted with measles during one month and his complicating pneumoniadid not develop until the following month, the pneumonia would be reported asoccurring in the former month, the date of the original admission. Thissimplifies the reading of the graphs as, for instance, the peak of measlesadmission and of the complicating pneumonias will thus appear in the same month.No allowance is necessary for the lapse of time between the development of theprimary disease and the onset of the complication.

As to the accuracy and completeness of the figures used inthe following pages, it must be said that doubtless many cases of pneumoniaescaped record in the monthly tables used, by reason of the fact that thedisease of record was taken to be the one given as the cause of the originaladmission. Complications and intercurrent diseases were included in the tablesof concurrent diseases, solely for enlisted men in the United States and Europe,and when complicating disease, not injury, but were not classified by months,except to some extent those occurring in influenza, and in measles. However, forthe present study the figures are very satisfactory and while doubtless manypneumonia cases were recorded under some other heading this error wasundoubtedly a nearly constant one and the important facts, the fluctuations inthe rates from month to month, are believed to be shown with substantialaccuracy.

EPIDEMIOLOGY

The history of epidemic influenza dates back to the dark agesof medicine and much of it is involved in the obscurity of uncertain diagnosis.It is outside the sphere of this chapter to consider this even in the mostcursory manner. Suffice it to say that the records of periodic visitations ofepidemic acute respiratory disease of such character as to be reasonablysupposed to have been influenza go back almost as far as does written history.These outbreaks have been of varying character and the descriptions sometimeslead to doubt as to the influenzal character of the disease. Some outbreaks wereassociated with large numbers of fatal cases of pneumonia while others equallywidespread were accompanied by relatively slight fatality. Before the pandemicof 1918 the

latest general outbreak of the disease was that of 1889-90,⁴involving very large numbers of cases but, compared to the more recent outbreak,a small loss of life. Even in this outbreak the fatality of the various wavesvaried greatly. Since that pandemic several minor outbreaks of less generaldistribution have occurred, notably in the winter of 1907-08⁴and in that of 1915-16.⁵ Neither of these led to enoughexcess mortality to make any considerable impression on the mortality curvesalthough it is evident on careful study.

The mortality figures for the United States registrationarea, 1911-1920,⁶ show a regularlyvarying curve for deaths from influenza and from the pneumonias, highest duringthe winter months, although at times the highest point was reached in March.(Chart V.) The highest monthly rates for influenza prior to 1918 were reached inthe winter of 1915-16, and were accompanied by some increase in the pneumoniadeath rates. This increased death rate for pneumonia continued with slightremission during the succeeding years up to the great pandemic of 1918, afterwhich the death rates for influenza for the whole area dropped back to normal. It was noted, however,that certain cities, New Orleans for example, showed an even higher death ratefor influenza in the winter of 1917-18 than had been the case two yearsearlier. There appears to have been then a certain possible increase in theprevalence of influenza and to a greater extent of pneumonia in the few yearspreceding the great outbreak of 1918.

For the Army, statistics are available since 1840, except forthe period of the Mexican War. Thereis little evidence in the records of any exceptional prevalence of influenzaduring the Civil War.

CHART V.-Comparative trends of mortality rates per1,000 for pneumonia and influenza, United States registrationarea for deaths, 1911-1920

From the end of the Civil War to the beginning of the WorldWar there was an almost uninterrupted slow decline in the admission rates forpneumonia in the Army.⁷This was broken only by two considerableelevations, one in 1888, the year before the outbreak of the 1889-90 influenzapandemic, and a second, almost as high, coinciding with the mobilization for theSpanish-American War in 1898, from 1913 there has been a very slight tendencyfor the rates to rise. The figures for influenza are less satisfactory owing tothe confusion as

to the exact diagnosis of the condition. Following the CivilWar relatively low rates prevailed for a few years to be succeeded by a periodof some 15 years when the reported rate ran nearly as high as that reachedduring 1890, when the peak of the pandemic reached the Army. Immediatelypreceding the pandemic, however, several years were recorded with as low a rateas was the case later. Following the 1889-90 pandemic the rates graduallydescended; low points were reached in 1902 and in 1914. The case fatality of thepneumonias was high during the Civil War and again reached almost the same pointin 1918. The high point in the intervening years was reached in 1887, afterwhich time there was a tendency to a gradual decline. In 1904, 1907, and 1915,all years in which influenza was noted as more than usually prevalent in thecivil population, there is a simultaneous rise. In the years previous to the1889-90 pandemic there appears to be no easily traceable relation betweenthem. With the exceptions of these relations, shown both in the civil andmilitary statistics between the incidence and the mortility for influenza andpneumonia, it would appear that the really significant fact brought out by thesefigures is that there is present at all times even in the interepidemic periodsa disease of such a character clinically as to lead large numbers of physicians,both in the Army and in civil practice, to call it influenza. Whether thisdisease is the same as that occurring in epidemic outbreaks remains for thefuture to decide, and the decision will be made when etiologic studies haveprogressed to the point that will render it possible to make a diagnosis basedon the identity of the inciting agent.

PREVALENCE AND IMPORTANCE DURING THEWAR PERIOD

General tables for the period of the war have been preparedshowing the total number of admissions and deaths from influenza, bronchitis,bronchopneumonia, and lobar pneumonia in the various racial groups comprisingthe Army, and for the different countries in which our troops were stationed.Rates have also been calculated for each of these groups based on a strengthwhich was obtained by the addition of the mean annual strengths for each of theyears of the war. The resulting rate is an average of the annual rates weightedfor the variations of strength from year to year. The figures thus obtainedpossess decided comparative value; they show the results that may be expectedfrom different races and in the different climates over a considerable periodof time in the presence of epidemic outbreaks of influenza. On the other hand,they are open to the objection that applies to all single figures purporting torepresent averages-the details are inevitably obscured. For the detailed studyof the epidemiology of these conditions the rates by months for the differentgroups are vastly preferable. These monthly rate are given in the study of theeffect of race and of length of service on the incidence and the mortality ofthe diseases under consideration. The consideration of these general tables,however, will serve to give an outline of the respiratory disease situation inthe Army during the World War, and certain general conclusions may be drawn fromthem as to the relative prevalence and fatality of these diseases in thedifferent groups.

TABLE 10.-Respiratory diseases. Primary admissions, officers and enlisted men, United States Army, by countries,
April 1, 1917, to December 31, 1919. Absolute numbers andannual ratios per 1,000 strength

	Influenza		Bronchitis		Bronchopneumonia		Lobar pneumonia		Total
	Absolute numbers	Annual ratios per 1,000	Absolute numbers	Annual ratios per 1,000	Absolute numbers	Annual ratios per 1,000	Absolute numbers	Annual ratios per 1,000	Absolute numbers	Annual ratios per 1,000
Total officers and enlisted men (including native troops)	791,907	191.82	255,148	61.80	32,572	7.89	45,774	11.09	1,125,401	272.60
Total officers and enlisted men, American troops	783,895	191.56	253,323	61.89	32,386	7.91	45,525	11.12	1,115,129	272.48
Total officers	28,621	138.68	11,876	57.54	1,021	4.95	975	4.72	42,493	205.89
Enlisted men, American:
White	671,322	186.50	214,561	59.61	24,422	6.78	31,903	8.86	942,208	261.75
Colored	59,448	207.46	20,045	69.95	4,825	16.84	11,482	40.07	95,800	334.32
Color not stated	24,504	---	6,841	---	2,118	---	1,165	---	34,628	---
Total	755,274	194.35	241,447	62.13	31,365	8.07	44,550	11.46	1,072,636	276.01
U.S. Army in the United Stated (including Alaska):
Officers	17,970	144.61	8,062	64.88	444	3.57	527	4.24	27,003	217.30
Enlisted men-
White	476,816	242.62	148,401	75.51	13,297	6.77	21,886	11.14	660,400	336.04
Colored	38,863	266.51	12,963	88.88	2,759	18.92	7,016	48.10	61,601	422.41
Total	515,679	244.27	161,364	76.43	16,056	7.61	28,902	13.69	722,001	342.00
Total officers and enlisted men	533,649	238.70	169,426	75.79	16,500	7.38	29,429	13.16	749,004	335.03
U.S. Army in Europe (excluding Russia):	9,743	132.15	3,517	47.40	552	7.49	424	5.75	14,236	193.09
Enlisted men-
White	176,240	119.92	60,098	40.89	10,761	7.32	9,000	6.12	256,099	174.25
Colored	18,619	152.10	6,681	54.58	1,986	16.32	4,149	33.89	31,435	256.89
Color not stated	23,859	---	6,679	---	2,100	---	1,076	---	33,714	---
Total	218,718	137.38	73,458	46.14	14,847	9.33	14,225	8.93	321,248	201.78
Total officers and enlisted men	228,461	137.15	76,975	46.21	15,399	9.24	14,649	8.79	355,484	201.39
Officers, other countries	908	108.25	297	35.41	25	2.98	24	2.86	1,254	149.50
Philippine Islands (including China):
White enlisted men	1,055	62.08	713	41.95	14	.82	49	2.88	1,831	107.73
Colored enlisted men	465	104.35	70	15.71	6	1.35	5	1.12	546	122.53
Total	1,520	70.86	783	36.50	20	.93	54	2.52	2,377	110.81
Hawaii:
White enlisted men	1,012	62.62	874	54.08	11	.68	58	3.59	1,955	120.97
Colored enlisted men	183	55.14	146	43.96	6	1.81	11	3.31	346	104.22
Total	1,195	61.35	1,020	52.41	17	.87	69	3.54	2,301	118.17
Panama: White enlisted men	3,272	166.18	866	43.99	7	.36	8	.41	4,153	210.94
Other countries:
White enlisted men	5,250	---	1,857	---	99	---	235	---	7,441	---
Colored enlisted men	127	---	30	---	12	---	34	---	203	---
Enlisted men, color not stated	587	---	153	---	11	---	17	---	768	---
Total	5,964	---	2,040	---	122	---	286	---	8,412	---
Transports:
White enlisted men	7,677	78.75	1,752	17.97	233	2.39	667	6.84	10,329	105.95
Colored enlisted men	1,191	113.05	155	14.71	56	5.32	267	25.34	1,669	158.42
Enlisted men, color not stated	58	---	9	---	7	---	72	---	146	---
Total	8,926	82.62	1,916	17.74	296	2.74	1,006	9.31	12,144	112.41
Native enlisted men:
Philippine Scouts	2,517	135.51	761	40.97	152	8.18	122	6.57	3,552	191.23
Hawaiians	1,052	187.35	230	40.96	5	.89	49	8.73	1,336	237.93
Porto Ricans	4,443	375.51	834	70.49	29	2.45	78	6.59	5,384	455.04
Total	8,012	222.40	1,825	50.66	186	5.16	249	6.19	10,272	284.41

Table 10 shows the absolute number of primary admissions forracial groups and for totals. Thus it is seen that influenza, bronchitis,bronchopneumonia, and lobar pneumonia were responsible for 1,125,401 primaryadmissions in the entire Army. Of these, influenza is credited with 791,907admissions; bronchitis, 255,148; bronchopneumonia, 32,572; lobar pneumonia,45,774. These diseases occurred also concurrently with, or secondarily to, otherdiseases or surgical conditions for which patients primarily were admitted tohospital, and were, in many instances, probably responsible for much of themortality which occurred and was otherwise reported. It is not now possible todiscover the total number of instances in which influenza, bronchitis,bronchopneumonia, and lobar pneumonia occurred concurrently with other diseasesor as complications of surgical conditions; however, partial results arepossible. Thus, it is possible to account for 798,509 cases of influenza,279,597 of bronchitis, 96,495 of bronchopneumonia, and 76,147 of lobarpneumonia. Allowing for the instances where diseases of this group complicatedother diseases of the same group, it is possible to account for 797,993 cases ofinfluenza, 272,735 of bronchitis, 37,334 of bronchopneumonia, and 51,115 oflobar pneumonia, a total of 1,159,177 cases of respiratory diseases; which, asstated above, can be only an approximation.

There were in all, during the World War, 3,515,464 admissionsto sick report for disease. Of these, 32 per cent were primarily for respiratorydisease, while an additional 0.96 per cent of the total suffered from thesediseases secondarily. The comparison of the annual ratios per thousand also isshown in Table 10. The total mean annual strength of the Army for the years 1917-1919 was4,128,479.⁸ It can be said,then, that 18.33 per cent, or 1 man to every 5.17, contracted influenza in theservice, 6.27 per cent, or 1 to every 15.14, contracted bronchitis, 0.86 percent, or 1 to 110.58, contracted bronchopneumonia, and 0.17 per cent, or 1 to80.77, contracted lobar pneumonia. The 1,159,177 cases of respiratory diseasesrepresent 26.63 per cent of the total number of men in the Army, or 1 to every3.5 men. Venereal disease was responsible for the next largest number ofadmissions (357,969), followed by mumps with 230,356 primary admissions andacute tonsillitis with 176,408.

As to group incidence, the figures show that the incidencewas in general higher among the American enlisted men (276.01) than amongofficers (205.89). The highest admission rates shown by any group was for theenlisted men from Porto Rico. Of their total rate of 455.04 per 1,000 forrespiratory diseases, however, 375.51 was for influenza. Their primaryadmissions for the pneumonias, especially for bronchopneumonia (2.45), wererelatively low. The next highest admission rate was shown by the coloredenlisted men in the United States (422.41). The colored rates were consistentlyhigher than those for the whites under the same conditions except for thecolored enlisted men in Hawaii, who had the lowest rate for total respiratorydisease (104.22) shown by group. The rate for the enlisted men of the PhilippineScouts (191.23) was lower than that of the Army as a whole (272.60), while thatfor the enlisted Hawaiians (237.93) was also below the average. The rate for thePhilippine Scouts (191.23) was higher than that for the white enlisted men inthe Philippine Islands (107.73) and also higher than that for colored troops(122.53) in the same territory.

In general, troops serving in the Tropics showed loweradmission rates than those in temperate climates. However, the rate for whiteenlisted men in Panama (210.94) was higher than the corresponding rate in Europe(174.25). The explanation of this is not forthcoming unless it be based on thefact that the troops in Europe had passed through the preliminary waves of theepidemic in the United States and had acquired an immunity which was notpossessed by the troops in Panama where the earlier waves of the influenzainvasion made little impression. This fact possibly accounts for the relativeimmunity of the troops in Europe as compared to the corresponding groups in theUnited States. It is not believed that the rates as given for men on transportsare comparable fairly with the others, since the difficulty of obtaining asatisfactory strength basis of computation or admission rate is insuperable. Thestrengths used appear to have been too high and the corresponding rates low. Itis generally admitted that during the fall wave of influenza (1918) theincidence and mortality on the transports was high, undoubtedly due to thenecessarily limited space available per man.

Among the military personnel during the World War there were44,270 deaths, occurring in cases having a primary diagnosis of influenza,bronchitis, bronchopneumonia, or lobar pneumonia.¹ Of these, 24,664 arecharged to influenza, 439 to bronchitis, 9,022 to bronchopneumonia, and 10,145to lobar pneumonia. There were, however, large numbers of cases of thesediseases reported, secondary to other diseases as previously stated. If to theseassociated cases we apply the same case fatality rates as shown by the primaryadmissions and deaths, we find that there were, in addition to the deaths givenabove, 189 from influenza, 30 from bronchitis, 1,319 from bronchopneumonia, and1,184 from lobar pneumonia. This method gives an estimated total of 24,853deaths from influenza as recorded, 469 from bronchitis, 10,341 frombronchopneumonia, and 11,329 from lobar pneumonia, a grand total of 46,992. Thisis nearly as large a total as that of the battle deaths, American ExpeditionaryForces-50,385.¹ The disease responsible for the next largest number ofdeaths was tuberculosis, as a primary admission, with 2,766, followed by measleswith 2,370 (also mainly due to pneumonia) and epidemic meningitis, with 1,836.

Of all the deaths charged to influenza, 99.4 per cent wererecorded as due secondarily to pneumonia, of which 66.1 per cent of the totalwere described as bronchopneumonia and 33.3 per cent as lobar pneumonia. Itappears that less than 1 per cent of the influenza deaths showed no recognizedsigns of pneumonia; that a very few cases are fatally overwhelmed by the primaryinfection appears probable. Of the deaths charged to bronchitis 84.5 per centwere recorded as secondarily due to pneumonia, of which 52 per cent were described as bronchopneumonia and 32.5per cent as lobar pneumonia, a proportion of 1.6 to 1 as compared with almostexactly 2 to 1 in the cases recorded as secondary to influenza. It is evidentthat, in so far as the bronchitis cases were of an influenzal nature, the caseswere evidently of a much milder average than those diagnosed frankly asinfluenza; the greater number of them, too, occurred in the earlier months ofthe war period. That this was largely true will be shown later. The men, then,who suffered from this infection in a form

severe enough to induce pneumonia and death were presumablythe more susceptible individuals. The proportion of lobar pneumonia tobronchopneumonia is seen to be higher in this group than in those franklydiagnosed as influenza. This agrees with the relations of these types ofpneumonia as noted for the white and colored races, in which the moresusceptible race showed a much larger relative proportion of lobar pneumoniaboth in admissions and in deaths.

The total deaths from disease during the war were 58,119, ofwhich those from respiratory diseases (as computed above) were 46,992, or 80.85per cent. Nearly all of these deaths from respiratory diseases, as has beenshown, were the result of pneumonia.

TABLE 11.- Respiratory diseases.Deaths, officers and enlisted men, United States Army, by countries, April 1,1917-December 31, 1919. Absolute numbers and ratios per 1,000
	Influenza		Bronchitis		Bronchopneumonia		Lobar pneumonia		Total
	Absolute numbers	Ratios per 1,000	Absolute numbers	Ratios per 1,000	Absolute numbers	Ratios per 1,000	Absolute numbers	Ratios per 1,000	Absolute numbers	Ratios per 1,000
Total officers and enlisted men (including native troops)	24,664	5.97	439	0.11	9,022	2.19	10,145	2.46	44,270	10.73
Total officers and enlisted men, American troops	24,575	6.00	439	.11	8,992	2.20	10,099	2.47	44,105	10.78
Total officers	596	2.89	22	.11	192	.93	194	.94	1,004	4.87
Enlisted men, American:
White	20,888	5.80	334	.09	6,480	1.80	7,073	1.96	34,775	9.75
Colored	2,287	7.98	42	.15	1,063	3.71	2,222	7.75	5,614	19.59
Color not stated	804	---	41	---	1,257	---	610	---	2,712	---
Total	23,979	6.17	417	.11	8,800	2.26	9,905	2.55	43,101	11.09
U.S. Army in the United States (including Alaska):
Officers	387	3.11	12	.10	80	.64	94	.76	573	4.61
Enlisted men-
White	14,617	7.44	24	.01	3,429	1.74	4,330	2.20	22,400	11.39
Colored	1,567	10.74	3	.02	634	4.35	1,363	9.35	3,567	24.46
Total	16,184	7.67	27	.01	4,063	1.92	5,693	2.70	25,967	12.30
Total officers and enlisted men	16,571	7.41	39	.02	4,143	1.85	5,787	2.59	26,540	11.87
U.S. Army in Europe (excluding Russia):
Officers	191	2.59	10	.14	106	1.44	93	1.26	400	5.43
Enlisted men-
White	5,753	3.91	304	.21	2,919	1.99	2,414	1.64	11,390	7.75
Colored	628	5.15	38	.31	395	3.23	778	6.36	1,839	15.05
Color not stated	794	---	40	---	1,244	---	534	---	2,612	---
Total	7,175	4.51	382	.24	4,558	2.86	3,726	2.34	15,841	9.95
Total officers and enlisted men	7,366	4.42	392	.24	4,664	2.80	3,819	2.29	16,241	9.75
Officers, other countries	18	2.15	---	---	6	.72	7	.83	31	3.70
Philippine Islands (including China):
White enlisted men	2	.12	1	.06	4	.24	7	.41	14	.83
Colored enlisted men	---	---	---	---	4	.90	1	.22	5	1.12
Total	2	.09	1	.05	8	.37	8	.37	19	.88
Hawaii:
White enlisted men	3	.06	---	---	---	---	3	.19	6	.25
Colored enlisted men	2	---	---	---	2	.60	1	.30	5	.90
Total	5	.06	---	---	2	.10	4	.21	11	.37
Panama: White enlisted men	9	.45	---	---	---	---	2	.10	11	.56
Other countries:
White enlisted men	137	---	3	---	48	---	54	---	242	---
Colored enlisted men	7	---	---	---	13	---	8	---	28	---
Enlisted men, color not stated	2	---	---	---	9	---	5	---	19	---
Total	146	---	3	---	70	---	67	---	286	---
Transports:
White enlisted men	367	3.76	2	.02	80	.83	263	2.70	712	7.31
Colored enlisted men	83	7.88	1	.09	15	1.42	71	6.74	170	16.13
Enlisted men, color not stated	8	---	1	---	4	---	71	---	84	---
Total	458	4.24	4	.04	99	.92	405	3.75	966	8.95
Native enlisted men:
Philippine Scouts	17	.92	---	---	27	1.45	25	1.35	69	3.72
Hawaiians	8	1.42	---	---	---	---	5	.89	13	2.31
Porto Ricans	64	5.41	---	---	3	.89	16	1.35	83	7.65
Total	89	---	---	---	30	---	46	---	165	---

TABLE 12.-Respiratory diseases. Officers and enlisted men, United States Army, by countries, April 1, 1917, to December 31, 1919. Case fatalities and ratios of bronchopneumonia to lobar pneumonia
	Case fatality					Ratio, bronchopneumonia to lobar pneumonia
	Influenza	Bronchitis	Broncho- pneumonia	Lobar pneumonia	Total	Admissions	Deaths
Total officers and enlisted men (including native troops)	3.1	0.17	27.7	22.2	3.9	0.71	0.89
Total officers and enlisted men, American troops	3.1	.17	27.7	22.2	4.0	.71	.89
Total officers	2.2	.18	18.8	19.9	2.4	1.05	.99
Enlisted men, American:
White	3.1	.16	26.5	22.2	3.7	.76	.91
Colored	3.8	.21	22.0	19.3	5.9	.42	.48
Color not stated	3.3	.60	59.3	52.3	7.8	1.82	2.06
Total	3.2	.17	28.1	22.2	4.0	.70	.89
U.S. Army in the United States (including Alaska):
Officers	2.2	.15	18.0	17.8	2.0	.84	.85
Enlisted men-
White	3.1	.16	35.8	19.8	3.4	.61	.79
Colored	4.0	.02	23.0	19.4	5.8	.39	.46
U.S. Army in Europe (excluding Russia):
Officers	2.0	.28	19.2	21.9	2.8	1.30	1.14
Enlisted men-
White	3.3	.51	27.1	26.8	4.4	1.19	1.21
Colored	3.4	.57	19.9	18.8	5.8	.49	.51
Officers in other countries	2.0	---	24.0	29.1	2.5	1.04	1.17
Philippine Islands:
White enlisted men	.19	.14	28.6	14.3	.8	.29	57
Colored enlisted men	---	---	66.6	20.0	.92	1.20	4.00
Hawaii:
White enlisted men	.30	---	---	5.2	.3	.19	---
Colored enlisted men	1.1	---	33.3	9.1	.9	.55	2.0
Panama: White enlisted men	.27	---	---	25.0	.27	.87	---

Of the race groups, the colored troops show consistently higher rates of death than the whites. The death rates for Porto Rican troops (7.65) is much lower than that of the total colored (19.59) and lower also than the total white rate (9.75). It about equals the rate observed for the white troops in Europe (7.75). In view of their high admission rate, their low fatality emphasizes

CHART VI.-Annual admission rates per 1,000 strength, white enlisted men in the United States, for influenza, bronchitis, bronchopneumonia, and lobar pneumonia, by months, April, 1917, to December, 1919

the influence of the climate in which a large proportion of these men were stationed. This applies also to the low rates for the Philippine Scouts (3.72) and the Hawaiians (2.31). The death rates for officers (4.87) were lower than for enlisted men (11.09). This difference is more marked in the United States than in Europe. The influence of environment is markedly shown in

the lower death rates for the tropical countries even in groups that show relatively high admission rates as in the case of white troops in Panama (0.56) and in that of the Porto Rican troops (7.65).

CHART VII.-The relations between the annual admission rates per 1,000 strength, white enlisted men in the United States, of the combined influenza and bronchitis and the combined lobar pneumonia and bronchopneumonia, by months, April, 1917, to December, 1919

In attempting to trace the various waves of the influenza epidemic, use has been made of the admission and death rates of influenza, bronchitis, bronchopneumonia, and lobar pneumonia, combined in each case into a single rate. From the number of admissions and the number of deaths, the case fatality has been calculated. Tables 13 to 22, inclusive, and Charts VIII to XIII show these factors for various specific groups of the Army. The epidemic prevalence of influenza in a certain month shows itself on the charts referred to in one or more of three ways, increases being in the admission rate, in the death rate, or in the case fatality. Differences in race or in length of service are accompanied by differences in the way in which a group reacts to the presence of influenza. When the epidemic is at its worst all three factors are

markedly affected, but in the case of milder waves only that factor to which the group responds most sensitively may show indications of the presence of the epidemic wave. Thus, in general, the colored race shows less variation in case fatality and more in morbidity than does the white race. So, too, the process of "seasoning" alters the relation of the case fatality and the admission rates to the death rates. With these preliminary facts in mind it is possible

CHART VIII.-Annual admission and death rates per 1,000 strength for white and colored enlisted men in the United States, total respiratory group of diseases, by months, April, 1917, to December, 1919

to examine the charts of annual rates by months and to determine with some degree of probability those months in which an epidemic wave had its occurrence. While statistics classified by weeks would be much more desirable for this purpose, such are not available, and the figures by months give a very clear-cut picture.

CHART IX.-Annual admission and death rates per 1,000 strength for white and colored enlisted men in Europe, total respiratory group of diseases, by months, June, 1917, to December, 1919. (No rates shown when strength was
less than 1,000)

CHART X.-Annual admission and death rates per 1,000 strength for white enlisted men in the United States and in Europe, total respiratory group of diseases, by months, April, 1917, to December, 1919

CHART XI.-Annual admission and death rates per 1,000 strength for colored enlisted men in the United States and in Europe, total respiratory group of diseases, by months, April, 1917, to December, 1919. (No rates shown when strength was less than 1,000)

CHART XII.-Case fatality rates (per cent), total respiratory group of diseases for white enlisted men in the United States and in Europe by months, April, 1917, to December, 1919

CHART XIII.-Case fatality rates (per cent), total respiratory group of diseases for colored enlisted men in the United States and in Europe, by months, July, 1917, to December, 1919

TABLE 13.-Total respiratory diseases (influenza, bronchitis, bronchopneumonia, and lobarpneumonia), white and colored enlisted men, United States Army, in the United States, bymonths, from April 1, 1917, to December 31, 1919
	Annual admission rates per 1,000 strength		Annual death rates per 1,000 strength		Case fatality rates (per cent)
	White	Colored	White	Colored	White	Colored
1917
April	141.97	96.07	2.16	4.92	1.52	5.12
May	113.07	125.53	1.91	---	1.67	---
June	59.53	27.84	.70	---	1.18	---
July	32.74	39.57	.26	1.80	.79	4.55
August	45.60	40.85	.17	1.41	.37	3.45
September	58.23	54.85	.23	2.55	.39	4.65
October	107.58	75.99	.98	2.75	.91	3.62
November	203.84	167.64	3.81	10.40	1.87	6.20
December	312.94	312.28	4.68	22.14	1.50	7.09
1918
January	344.58	308.41	5.83	16.10	1.69	5.22
February	200.08	226.54	2.48	10.81	1.24	4.77
March	363.91	582.10	5.22	26.71	1.43	4.59
April	493.20	649.87	4.63	34.16	.94	5.26
May	167.51	368.70	1.54	16.57	.92	4.49
June	85.94	112.62	.84	4.98	.98	4.42
July	73.13	145.67	.93	5.95	1.27	4.08
August	83.05	197.79	1.27	10.13	1.53	5.12
September	974.36	896.71	54.06	53.86	5.55	6.01
October	1,978.15	1,584.95	95.69	111.66	4.84	7.05
November	296.27	172.86	9.74	6.37	3.29	3.69
December	266.65	191.41	9.08	6.92	3.41	3.62
1919
January	271.90	236.39	9.14	8.09	3.36	3.42
February	138.01	163.75	3.87	4.73	2.80	2.89
March	118.13	134.14	2.30	5.38	1.95	4.01
April	89.37	80.08	1.09	1.61	1.22	2.01
May	65.84	72.73	.79	1.74	1.20	2.39
June	54.24	65.28	.43	1.94	.79	2.97
July	49.99	112.46	.50	1.80	1.00	1.60
August	58.46	29.98	.39	.67	.67	2.23
September	59.46	39.20	.33	1.06	.55	2.70
October	63.40	30.38	.26	1.32	.41	4.35
November	69.42	39.55	.18	1.36	.26	3.44
December	74.78	61.75	.62	---	.83	---

TABLE 14.-Total respiratory diseases (influenza, bronchitis, bronchopneumonia, and lobarpneumonia), white and colored enlisted men, United States Army in Europe, by months, from June, 1917, to December 31, 1919
	Annual admission rates per 1,000 strength		Annual death rates per 1,000 strength		Case fatality rates (per cent)
	White	Colored	White	Colored	White	Colored
1917
June	24.15	---	0.89	---	3.69	---
July	57.04	---	.84	---	1.47	---
August	57.07	---	.24	---	.42	---
September	65.92	---	.17	---	.26	---
October	153.04	---	1.82	---	1.19	---
November	311.49	733.67	1.94	---	.62	---
December	407.17	733.18	2.76	31.39	.68	4.28
1918
January	209.50	969.57	3.60	51.17	1.72	5.28
February	111.70	621.11	1.56	19.87	1.40	3.20
March	125.52	725.57	2.00	33.27	1.59	4.59
April	108.81	662.88	.89	15.15	.82	2.29
May	109.90	421.67	.91	8.06	.83	1.91
June	102.70	265.64	.76	8.67	.74	3.26
July	58.94	189.27	.78	6.10	1.32	3.22
August	73.14	130.62	2.02	7.45	2.76	5.70
September	263.80	473.05	21.05	50.48	7.98	10.67
October	397.00	408.51	33.52	50.75	8.44	12.42
November	200.66	272.07	6.19	11.78	3.08	4.33
December	207.91	266.26	4.42	10.52	2.13	3.95
1919
January	171.32	241.63	5.55	10.77	3.24	4.46
February	223.48	283.59	9.18	12.35	4.11	4.35
March	108.28	160.38	2.23	3.41	2.06	2.13
April	65.84	129.51	.93	2.39	1.41	1.85
May	39.68	68.92	.47	1.09	1.18	1.58
June	30.13	65.64	.71	1.50	2.36	2.29
July	46.32	76.77	.97	1.06	2.09	1.38
August	76.24	82.76	.75	---	.98	---
September	48.69	37.39	---	---	---	---
October	88.89	---	.57	---	.64	---
November	128.73	---	.63	---	.49	---
December	371.41	---	7.19	---	1.94	---

NOTE.-No rates given when strength less than 1,000.

TABLE 15.-Annual admission rates per 1,000 strength, white enlisted men, in the United States, by months, from April 1, 1917, to December 31, 1919
	Strength	Influenza (all)	Bronchitis	Bronchopneumonia	Lobar pneumonia
1917
April	183,758	56.03	71.64	2.61	11.69
May	245,454	47.08	55.87	1.66	8.46
June	309,205	21.00	33.26	.85	4.42
July	458,817	10.41	19.25	.39	2.69
August	562,714	14.65	27.77	.66	2.52
September	776,466	20.25	33.72	.94	3.32
October	1,032,244	40.55	55.67	1.62	9.74
November	1,061,422	78.47	100.70	4.44	20.23
December	1,129,065	127.43	157.05	6.23	22.23
1918
January	1,096,434	147.91	161.38	4.51	30.78
February	1,095,039	101.43	78.15	2.18	18.32
March	1,129,223	224.48	117.47	2.91	19.05
April	1,168,558	366.74	106.18	3.31	16.97
May	1,197,757	102.36	56.97	1.79	6.39
June	1,303,746	42.50	37.69	1.27	4.48
July	1,328,513	34.76	33.57	.93	3.87
August	1,284,247	37.32	40.04	1.72	3.97
September	1,321,440	869.54	71.72	14.49	18.61
October	1,343,933	1,823.09	84.77	45.70	24.59
November	1,255,195	203.97	75.26	13.75	3.29
December	941,219	155.26	96.23	10.49	4.67
1919
January	672,937	139.49	116.61	10.95	4.85
February	471,815	59.62	69.97	4.58	3.84
March	406,839	45.57	64.71	4.16	3.69
April	339,836	33.16	49.86	3.28	3.07
May	291,810	16.82	44.38	2.30	2.34
June	246,903	12.73	38.15	1.56	1.80
July	215,104	7.31	38.72	1.28	2.68
August	156,791	8.57	47.14	1.07	1.68
September	149,360	8.68	49.18	.80	.80
October	139,877	10.12	51.82	.60	.86
November	132,403	5.17	62.80	1.00	.45
December	135,441	5.05	67.25	.80	1.68

TABLE 16.-Annual death rates per 1,000 strength, white enlisted men, in the United States, by months, from April 1, 1917, to December 31, 1919
	Influenza (all)	Bronchitis	Broncho- pneumonia	Lobar pneumonia		Influenza (all)	Bronchitis	Broncho- pneumonia	Lobar pneumonia
1917					1918
April	0.07	---	0.46	1.63	September	41.72	0.03	5.19	7.12
May	.05	---	.20	1.66	October	73.26	.02	14.16	8.25
June	---	---	.16	.54	November	6.88	.06	2.05	.75
July	---	---	.10	.16	December	5.58	.08	2.40	1.02
August	---	---	.06	.11	1919
September	---	---	.06	.17	January	4.96	---	3.00	1.18
October	---	---	.12	.86	February	2.14	---	1.02	.71
November	.01	0.01	.90	2.89	March	.91	---	.68	.71
December	.14	.02	1.24	3.28	April	.42	---	.35	.32
1918					May	.21	---	.29	.29
January	.22	.01	1.04	4.56	June	---	---	.24	.19
February	.07	---	.42	1.99	July	.11	---	.17	.22
March	.38	---	.58	4.26	August	.08	---	.31	---
April	.45	.01	.79	3.38	September	.08	---	.08	.17
May	.23	---	.33	.98	October	.09	---	.17	---
June	.17	---	.18	.49	November	---	---	.09	.09
July	.37	.01	.14	.41	December	---	.09	.18	.35
August	.44	---	.31	.52	December	---	.09	.18	.35

TABLE 17.-Annual admission rates per 1,000 strength, colored enlisted men, in the United States, by months, from April 1, 1917, to December 31, 1919
	Strength	Influenza (all)	Bronchitis	Bronchopneumonia	Lobar pneumonia
1917
April	4,870	24.63	56.66	2.46	12.32
May	5,826	16.46	96.72	2.06	10.29
June	5,171	2.32	13.92	2.32	9.28
July	6,675	12.59	23.38	---	3.60
August	8,519	2.82	18.31	---	19.72
September	9,409	11.48	21.68	1.28	20.41
October	21,795	15.97	35.79	2.75	21.48
November	39,225	26.31	56.90	9.18	75.25
December	36,851	50.80	126.67	18.56	116.25
1918
January	50,705	87.81	124.03	6.63	89.94
February	49,955	70.87	90.32	6.97	58.38
March	54,814	257.01	209.50	12.04	103.55
April	59,015	269.02	246.85	13.62	120.38
May	87,650	108.01	166.48	13.01	81.20
June	89,305	30.24	50.93	4.17	27.28
July	124,976	25.54	83.06	4.61	32.46
August	168,422	51.44	93.98	5.56	46.81
September	164,846	713.91	109.49	22.57	50.74
October	182,705	1,372.54	56.02	85.65	70.74
November	150,587	79.94	51.16	16.10	25.66
December	104,140	78.12	69.26	16.83	27.20
1919
January	68,337	86.76	103.09	19.14	27.40
February	66,104	49.74	74.07	12.71	27.23
March	44,634	30.38	65.86	12.63	25.27
April	29,824	16.90	47.89	4.83	10.46
May	20,780	19.05	44.45	3.46	5.77
June	18,562	12.28	37.49	5.17	10.34
July	20,058	8.37	68.80	7.18	28.11
August	18,013	4.66	21.32	2.00	2.00
September	11,322	5.30	32.84	---	1.06
October	9,084	1.32	27.74	---	1.32
November	8,792	---	34.10	1.36	4.09
December	8,935	1.34	48.33	---	12.08

TABLE 18.-Annual death rates per 1,000 strength, colored enlisted men in the United States, by months, from April 1, 1917, to December 31, 1919
	Influenza (all)	Bronchitis	Broncho- pneumonia	Lobar pneumonia		Influenza (all)	Bronchitis	Broncho- pneumonia	Lobar pneumonia
1917					1918
April	---	---	2.46	2.46	September	35.01	0.07	7.28	11.50
May	---	---	---	---	October	66.14	---	22.20	23.32
June	---	---	---	---	November	1.51	---	2.15	2.71
July	---	---	---	1.80	December	1.27	.23	2.19	3.23
August	---	---	---	1.41	1919
September	---	---	---	2.55	January	1.76	---	2.99	3.34
October	---	---	.55	2.20	February	.55	---	.91	3.27
November	---	---	1.22	9.18	March	.81	---	2.69	1.88
December	---	---	1.30	20.84	April	---	---	---	1.61
1918					May	.58	---	.58	.58
January	---	---	2.13	13.97	June	---	---	---	1.94
February	0.24	---	1.68	8.89	July	---	---	.60	1.20
March	.44	---	3.94	22.33	August	---	---	.67	---
April	1.63	---	5.08	27.45	September	---	---	---	1.06
May	.27	---	2.74	13.56	October	---	---	---	1.32
June	.27	---	.54	4.17	November	---	---	1.36	---
July	.38	---	.77	4.80	December	---	---	---	---
August	.93	---	.93	8.27	December	---	---	---	---

TABLE 19.-Annual admission rates per 1,000 strength, white enlisted men in Europe, by months, from June, 1917, to December 31, 1919
	Strength	Influenza (all)	Bronchitis	Broncho- pneumonia	Lobar pneumonia
1917
June, and preceding	13,420	9.84	8.94	1.79	3.58
July	28,821	15.82	34.97	1.67	4.58
August	50,882	19.10	35.14	1.89	.94
September	70,266	23.74	38.42	1.54	2.22
October	92,139	58.35	84.79	5.60	4.30
November	123,429	146.12	153.02	4.57	7.78
December	160,178	172.46	219.21	5.69	9.81
1918
January	193,264	96.80	92.52	4.84	15.34
February	223,130	57.60	46.68	1.77	5.65
March	283,268	66.55	44.44	3.26	11.27
April	388,048	66.39	33.27	1.70	7.45
May	587,240	79.37	24.46	1.06	5.01
June	796,427	83.73	14.89	1.10	2.98
July	1,063,192	45.84	9.76	.77	2.57
August	1,266,592	56.41	12.35	1.72	2.66
September	1,527,793	214.90	27.77	12.04	9.09
October	1,635,321	315.04	43.84	23.58	14.54
November	1,682,836	146.82	41.59	7.49	4.76
December	1,591,962	136.07	59.32	6.96	5.56
1919
January	1,488,683	102.44	56.45	7.63	4.80
February	1,310,083	137.20	65.12	12.50	8.66
March	1,115,693	58.26	41.18	5.24	3.60
April	853,425	30.36	29.04	3.21	3.23
May	569,842	18.76	16.83	1.92	2.17
June	271,633	11.97	15.24	1.06	1.86
July	111,634	17.41	23.65	.75	4.51
August	48,006	15.00	54.74	2.00	4.50
September	30,315	15.04	27.32	2.77	3.56
October	21,055	18.23	64.96	3.99	1.71
November	18,920	10.78	110.34	2.54	5.07
December	18,379	90.08	267.62	9.14	4.57

TABLE 20.-Annual death rates per 1,000 strength, white enlisted men in Europe, by months, from June, 1917, to December 31, 1919
	Influenza (all)	Bronchitis	Broncho- pneumonia	Lobar pneumonia		Influenza (all)	Bronchitis	Broncho- pneumonia	Lobar pneumonia
1917					1918-Continued
June and preceding	---	---	---	0.89	October	18.04	.68	8.64	6.16
July	---	---	0.42	.42	November	3.17	.46	1.36	1.20
August	---	---	.24	---	December	2.19	.32	.93	.98
September	---	---	---	.17	1919
October	---	---	1.17	.65	January	2.39	---	2.18	.98
November	---	---	.97	.97	February	4.19	---	2.94	2.05
December	0.22	0.22	1.12	1.20	March	.83	---	.86	.54
1918					April	.30	---	.31	.32
January	.25	.25	.99	2.11	May	.11	---	.21	.15
February	---	.16	.32	1.08	June	.09	---	.13	.49
March	.13	.04	.30	1.53	July	.11	---	.11	.75
April	.15	---	.06	.68	August	---	---	.50	.25
May	.16	.12	.06	.57	September	---	---	---	---
June	.24	.05	.09	.38	October	.57	---	---	---
July	.25	.03	.09	.41	November	---	---	---	.63
August	.83	.05	.42	.72	December	3.26	1.31	1.31	1.31
September	12.05	.59	4.45	3.96	December	3.26	1.31	1.31	1.31

TABLE 21.-Annual admission rates per 1,000 strength, colored enlisted men, in Europe, by months, from November, 1917, to September 30, 1919
	Strength	Influenza (all)	Bronchitis	Bronchopneumonia	Lobar pneumonia
1917
November	2,392	125.63	572.86	20.10	15.08
December	5,346	159.19	426.01	22.42	125.56
1918
January	8,673	361.00	278.01	23.51	307.05
February	9,664	196.27	268.32	18.63	137.89
March	11,541	225.57	291.06	23.91	185.03
April	12,667	312.50	227.27	7.58	115.53
May	28,279	224.38	117.10	8.06	72.13
June	33,208	104.45	104.45	7.59	49.15
July	47,171	80.13	55.46	6.36	47.32
August	78,734	73.77	27.13	5.94	23.78
September	91,270	332.63	56.93	32.08	51.41
October	138,827	288.01	35.18	38.12	47.20
November	148,679	194.51	31.72	16.30	29.54
December	148,372	171.63	48.69	17.87	28.07
1919
January	140,396	154.02	43.25	17.86	26.50
February	131,219	156.10	68.50	22.04	36.95
March	123,152	95.29	41.02	8.67	15.40
April	119,801	64.31	43.27	8.81	13.12
May	108,650	23.53	33.24	3.98	8.17
June	64,166	21.69	30.11	3.18	10.66
July	12,508	16.31	47.02	3.84	9.60
August	1,714	6.90	62.07	---	13.79
September	1,287	9.35	18.69	9.35	---
NOTE.-No rates given when strength less than 1,000.

TABLE 22.-Annual death rates per 1,000 strength, colored enlisted men in Europe, by months, from November 1, 1917, to September 30, 1919
	Influenza (all)	Bronchitis	Broncho- pneumonia	Lobar pneumonia		Influenza (all)	Bronchitis	Broncho- pneumonia	Lobar pneumonia
1917					1918-Continued
November	---	---	---	---	November	4.68	.24	2.34	4.52
December	---	---	2.24	29.15	December	3.80	.24	2.35	4.13
1918					1919
January	---	4.15	1.38	45.64	January	2.99	---	3.59	4.19
February	---	1.24	2.48	16.15	February	3.29	---	2.93	6.13
March	---	2.08	1.04	30.15	March	.88	---	.97	1.56
April	0.95	1.89	---	12.31	April	.29	---	1.10	1.00
May	1.27	.85	.85	5.09	May	.10	---	.11	.88
June	.72	---	.72	7.23	June	---	---	.75	.75
July	.51	.25	.25	5.09	July	.10	---	.96	---
August	.91	.30	.91	5.33	August	---	---	---	---
September	23.53	1.05	8.68	17.22	September	---	---	---	---
October	21.09	.95	12.37	16.34	September	---	---	---	---
NOTE.-No rates given when strength less than 1,000.

Chart VIII shows that the admission and death rates were both high for white enlisted men in the United States in April, 1917; not as high as in the subsequent April, but in relation to the low rates of the following summer, the difference is almost as marked. Was this high point due to influenza at that early date, or was it merely an unusual seasonal prevalence of respiratory disease? The colored troops had the same high point at this time, and it is to be remembered that in April, 1917, the number of recruits in the Army was comparatively small; in the main it consisted of seasoned troops. Consequently any considerable rise in rates at this time is of more significance than would be the case had a large proportion of recruits recently joined. The case fatality

rate for white enlisted men at this time was exceeded only twice in the subsequent months, during the great wave of the fall, and in the first recurrence thereafter. It seems justified to conclude that we have here fairly definite evidence to the effect that the influenzal infection was then at work among the troops. The first recognizable wave then during the war period occurred in the first month shown in the statistics. Following this wave there was a sharp drop to a level lower than ever subsequently reached during the war. The next well-marked rise reached its height in December, 1917, and January, 1918, varying somewhat with different localities, the total figure highest in January for admissions and deaths in this country and for admissions in Europe. The peak month for deaths at this time in Europe was December.

The month of February showed a well-marked decline in the rates for all groups, to be followed by a third and, in this country, much higher peak in March and April, deaths higher in March and admissions in April. This wave shows on the curves for troops in Europe, but not nearly as sharply as in the United States. The admission rates rise slightly in March and then fall very slowly for the succeeding months, apparently continuing high through June. This was the period of the type of influenza known to the American Expeditionary Forces as "three day fever." Except in the first month it was accompanied by relatively little mortality. The next rise shown is that culminating in the great October peak. There is in all groups studied a higher admission and death rate for respiratory diseases in August than there was for July. This is a significant phenomenon taken in connection with the fact that August is usually expected to show the lowest rates for these diseases of the entire year. In the colored troops in the United States this rise is seen to begin a month earlier still, or in July, 1918. At this time, a large number of cases of rather fatal lobar pneumonia were noted in this race in widely scattered camps. It seems probable from the progressive character of the subsequent rise in the rates that these cases represent the earliest of the great wave, occurring in the most susceptible group in the Army. The case fatality for white troops also showed a rise in the month of July and progressively continued into the high point of the fall wave. To determine the actual time of beginning of the rise for this wave recourse is had to a compilation of the weekly telegraphic reports of current medical statistics. Though these are not satisfactory for the purpose of estimating the total number of cases, they do show well the comparative numbers from week to week. Influenza was not on the list of diseases of which weekly reports were required at that time, and so the compilation includes only those stations that did report the disease. The data are assembled in Table 23. There is seen to be a progressive increase in cases reported as influenza beginning with the week ending August 4, 1918, and of the influenzal pneumonia cases beginning with the week ending August 18 of that year. If this was really the beginning of the great epidemic wave we should expect that if these series of data were plotted out on a logarithmic scale the increase from week to week would plot out as a straight line following the usual logarithmic rise of an epidemic curve. That this condition is substantially fulfilled is seen in Chart XIV.

TABLE 23.-Incidence of influenza and of influenzal pneumonia, by weeks, June 17, 1918, to December 29, 1918. Annual admission rates per 1,000 strength
Week ending-	Influenza	Influenzal pneumonia	Week ending-	Influenza	Influenzal pneumonia	Week ending-	Influenza	Influenzal pneumonia	Week ending-	Influenza	Influenzal pneumonia
June 23	167.7	20.1	Aug. 11	183.8	12.4	Sept. 29	1,700.0	85.2	Nov. 17	570.0	24.3
June 30	185.0	18.7	Aug. 18	206.4	13.5	Oct. 6	1,702.0	77.0	Nov. 24	653.0	27.4
July 7	153.2	14.4	Aug. 25	270.0	17.5	Oct. 13	1,565.0	74.7	Dec. 1	769.0	32.8
July 14	171.6	15.5	Sept. 1	408.0	24.7	Oct. 20	1,880.0	86.5	Dec. 8	800.0	34.4
July 21	155.9	13.0	Sept. 8	467.0	26.9	Oct. 27	1,351.0	61.1	Dec. 15	914.0	38.8
July 28	134.0	13.9	Sept. 15	683.0	38.4	Nov. 3	1,248.0	55.4	Dec. 22	647.0	28.4
Aug. 4	173.2	12.5	Sept. 22	1,097.0	56.5	Nov. 10	816.0	35.9	Dec. 29	506.0	22.3

It is evident that both in this country and in Europe the rates for respiratory disease began to rise at least as early as the month of August and that the rise was practically simultaneous in the two forces, separated by thousands of miles of water. That an epidemic wave once developed is spread by contact of cases, is of course, incontrovertible. But that the widespread, practically simultaneous, increase in the rates that was observed not only with this wave but also with all the preliminary and recurrent waves of the pandemic could have been accounted for by transmission from case to case of a common source seems incredible.

The rise for the October peak, then, began about the first ofAugust; the decline after the peak was prompt, but the admission rates remainedrelatively high during the winter of 1918-19, though not in this country or inEurope equaling the rates observed for the previous winter. There was, inJanuary and February, evidence of a recurrent wave thataffected the troops both in this country and in France, the peak abroad comingsomewhat later than that here. The fatality of this recurrence was lower in thiscountry than it was abroad, as indeed was the case with the peak wave ofOctober. There is also definite evidence in allthe charts of a distinct minor increase of activity of the influenza virusin the months of June and July, 1919. This isreflected to some extent in almost all therates, but shows most distinctly in the admission rates of the colored troopsand in the case fatality in thewhites, thus harmonizing with the reactionof seasoned troops of those races to the action of the disease. The final wave shown during the arbitrarilylimited period of this study is the beginning of the so-called1920 recurrence, which reached its height in January and February of that year,but is distinctly shown on the charts in its incipiency in December, 1919.

It appears from this summary that evidences of epidemic waves of influenza during the war period were noted for: (1) April, 1917; (2) December, 1917, to January, 1918; (3) March to April, 1918; (4) September to October, 1918; (5) January to February, 1919; (6) June to July, 1919; (7) The beginning of of the 1920 recurrence observed in December, 1919.

Of these waves, the first, second, fourth, sixth, and seventh (culminating as it did a couple of months later) fall at intervals that coincide very well with the 33-week period of influenza pointed out by Brownlee.⁹The third and fifth waves, which do not fit into this scheme, also fall about that time apart. The Army Medical Department statistics of the World War are not particularly

well adapted to the study of periodicity in its most exact form, since these statistics cover too large an assemblage of widely separated units; however, the distinct approximation to the period of Brownlee is of more than passing interest. The point of greatest importance to be brought out at this time is the very evident fact that the time relations of the various waves described in the troops at home and abroad seem to preclude the idea of the transmission of these waves from one command to another. The peak months varied in some instances but the evidences of the beginning of the rise for each wave were usually coincident in the two groups. This observation is of fundamental importance in the epidemiology of the disease.

CHART XIV.-Incidence of influenza and influenzal pneumonia, by weeks, for certain camps inthe United States, June 17 to December 29, 1918. Annual admission rates per 1,000 strength based on weekly reports of current medical statistics

Epidemiologists have usually approached the study of epidemicinfluenza from the point of view that the disease had originated in some distantland and reached any given point by certain fairly definite routes. The MedicalDepartment figures, as summarized above, show that the fatal wave of the fallof 1918 was preceded by several preliminary outbreaks, andfollowed by a number of recurrences, and that the rates of incidence andmortality for widely separated commands were so nearly synchronous in their riseand fall for each wave as to impress the student with the probability that thevirus of the disease had achieved a world wide distribution months before themortality records forced recognition of its prevalence. It must follow from this observation that the disease onceestablished in a community passes through alternating phases of increasing anddecreasing activity, due either to changing qualities of the virus itself or tovariations in the susceptibility of the population. And here again we are led tospeculate as to the possibility that this disease may in fact be constantlypresent in all populations, making its presence felt only through cases of suchmildness as to attract little attention unless such cases are indeed the usualpredisposing condition needful for the production of the endemic pneumonias ofthe interepidemic periods.

COMPARISON OF ARMY AND CIVIL DEATH RATES

The high incidence and mortality from pneumonia in theearlier days of mobilization, together with the explosive character of the fallepidemic (1918), as it appeared in the camps in this country, with the appallingnumber of deaths concentrated in a very few weeks in each camp, have led to theidea that the death rate from respiratory disease was enormously higher in theArmy than was the case in civil life. While this was true, to some extent,especially as regards the newly recruited troops in the camps in this country,it is believed that the actual figures do not bear out the general impression.The closely knitted communities of the camps afforded the greatest opportunityfor the epidemic to spread, but while the rates in these camps were higher thanthose of civil life, they were not as high as some apparently have believed.

The comparison of death rates in the Army and in civil lifeis rather a complicated matter and many factors must be taken into considerationbefore fairly comparative figures can be adduced. In the first place, theusually published rate for a civil community is a gross one, taking intoconsideration no difference in the specific rates for race, age, and sex. TheArmy rate, on the contrary, is based on a population exclusively male and of anage grouping quite different from that of a civil community. For presentpurposes it may fairly be assumed that for the period of greatest mortalityduring the World War the proportion of colored and white troops in the servicewas the same as that in the corresponding age groups of the general population.In order to make a comparison, then, it becomes necessary to apply the specificrates for the various age groups of males in the civil population to thestrength of the corresponding age groups in the Army, and to compare the numberof deaths thus arrived at with the number that actually occurred in the militaryservice. For the purpose of comparison, the year 1918 has been selected, as thiswas the time of the greatest strength of the Army, also because for that periodmonthly figures are available on which to base the rate calculation for

the Army. These monthly compilations, however, have been made only for enlisted men in the United States and Europe and the comparison, therefore, is based on data which show a rate somewhat higher than that of the whole Army, which included officers, and also enlisted men in various tropical stations, both of which classes showed a decidedly lower death rate than did the enlisted men in this country and in Europe. Figures, by months, for the entire Army, however, are not available, and inasmuch as the figures used include the great majority of the Army, the comparison seems a fair one.

The proportion of men of different ages in the Army during the war is not a matter of direct record; however, from the records of the War Risk Insurance Bureau it is possible to show the ages of 3,673,125 men, obtained from their applications for policies of Government insurance. Since it is fair to assume that the average period of service was in the neighborhood of one year, and that the insurance policy was taken out at or near the beginning of the service period, the ages of these men have been advanced six months each to obtain an average figure for the war period. The following table shows the age grouping thus obtained.

Age group	Number of men	Percentage of total	Age group	Number of men	Percentage of total
15-19 years	138,267	3.76	35-39 years	54,975	1.50
20-24 years	1,644,952	44.80	40-44 years	24,975	.68
25-29 years	1,345,679	36.60	45-49 years	9,990	.27
30-34 years	448,631	12.20	50-64 years	5,656	.15

It is believed that these figures are fairly representative of the actual conditions, although the fact that officers as well as enlisted men are included somewhat increases the percentage of the older age groups.

It has been shown that when the strength of a command variesgreatly from month to month, and at the same time the death rates vary greatly,a death rate based on the total number of deaths for the year and the meanannual strength will not give a figure fairly comparable to that of a command ofnearly uniform strength having the same monthly death rates.¹⁰ Ifit happens that the months of greatest strength are also those of the highestdeath rates, the effect on the annual death rate, calculated as above stated, isto make it too high, while, if the high death rates coincide with a period oflow strength, the rate will be too low as compared with a command of uniformstrength having the same monthly rates. The average of the monthly rates for theyear, however, gives a truly comparable figure whatever variations there may beon the part of the strength or of the monthly death rates. In comparing themilitary and civil rates, then, it is important to take this factor intoconsideration, for while the Army increased to a maximum strength in the monthsof the highest death rates due to the influenza epidemic, the number of males ofmilitary age who remained at home and contributed to the civil death ratescorrespondingly diminished. These fluctuations were great enough to have a verymarked effect on the rates. The rates for months but not by age groups areavailable for the Army. The rate for the age groups but not by months areavailable for the civil population (registration States). Hence some method mustbe devised to reduce the two sets of data to a common basis.

The average of the monthly death rates from disease of all kinds for enlisted men in the United States and Europe was 16.1 per thousand.¹The total number of deaths from disease was 44,924.¹ If this total number of deaths during the year be divided by the death rate and multiplied by one thousand it will give a figure representing the strength that would have given the same number of deaths during the year had the strength and the death rate remained uniform throughout the year. The figure in this case is 2,788,000. This is an average of the monthly strengths weighted by the death rates for the corresponding months. For the reasons given above this is the best figure to use for the average strength of the Army when comparing its rates with those of the civil population.

The next factor to be estimated is the proportion of the various age groups of the male population as estimated for 1918 that were not in the services (Army, Navy, and Marine Corps) and so remained to contribute to the civil rate. The usual arithmetical method of estimating population based on the census returns of 1910 and of 1920 can not be applied to the years previous to 1919 without taking into consideration the excess deaths in the various age groups due to the influenza epidemic and to battle casualties. If these be estimated as carefully as possible and added to the population as found in 1920 the estimate for 1918 becomes satisfactory. If, then, from the population of each age group, as thus estimated, is subtracted the number of men of each age in the service the remainder will represent the number in each age group that contributed to the civilian death rates. It is estimated that the Army represented 87.5 of the total military forces during the war, the balance being the Navy and the Marine Corps. Hence the weighted mean strength of the services during 1918 was 3,180,000 men. The registration States furnished 76.51 per cent of this total or 2,435,000. If these men be divided into age groups according to the percentages shown in the table of ages the results will represent the number of men in each age group furnished to the services by the registration States. These numbers subtracted from the estimated population of the registration area in each group will give the civil population in each group during 1918, and from these last figures the death rates for each group may be calculated from the number of deaths in each group in the registration states as furnished by the Census Bureau. The following specific death rates from disease for males of the indicated age groups were arrived at by the method given.

Age	Death rate from disease	Age	Death rate from disease
15-19 years	7.05	35-39 years	14.12
20-24 years	13.18	40-44 years	13.23
25-29 years	17.50	45-49 years	14.03
30-34 years	17.78	50-64 years	22.58

If these rates are applied to the weighted mean strength of the Army and the latter divided into age groups according to the age table given, the total result is 42,184 deaths, or 15.1 per thousand on the weighted mean strength.

The comparative rate for the Army as given above was 16.1, a difference that, if occurring from one year to the next in the same population, would not be regarded as highly significant.

When, however, the rates for the total respiratory diseases are treated in the same manner the rates obtained are 12.59 for the Army and 9.96 for the civil population. It is evident that the Army had a decidedly higher reported death rate for the respiratory diseases than was found in civil life and equally evident that the reverse was true for other causes of death. In fact, the figures indicate that at civil rates the Army would have lost nearly seven thousand more men than was actually the case from causes other than the respiratory diseases.

The reasons for these differences are probably several. It will be shown later that men from rural districts, when introduced into the conditions of barrack life, are much more susceptible to respiratory infections than are their urban brethren. Urban rates for respiratory diseases are always markedly higher than those of rural districts. In the Army hundreds of thousands of young men from rural districts were living in the Army camps in conditions of closer contact than in ordinary city life. The majority of these men in this country at the time of the great epidemic were comparatively new to the service, 76 per cent had seen less than four months' service,^a and had little or no time for "seasoning." Certain cities in the country showed rates during the epidemic higher than those of the Army. For the last four months of 1918, had the rates that obtained in Philadelphia been applied to the Army strength there would have been 39,894 deaths from disease and 33,287 from the respiratory diseases as compared with 36,858 and 33,136, respectively, which actually occurred in the Army. Again, there is seen the greatly larger number of deaths from causes other than the respiratory diseases occurring in the civil population even during the epidemic months. So great a disparity suggests some differences in the standards of diagnosis in the two sets of figures. That many deaths occurring in the registration area were really due to influenza or pneumonia while otherwise reported, is suggested by the notable and unseasonable increases during the epidemic period in deaths from a number of other causes. It is undoubtedly true also that during this period, the country over, a great number of deaths occurred that were never reported. The conditions in many places were such that the keeping of accurate records was an impossibility. In the Army, however, every man had to be accounted for and the death records are as nearly accurate as it is possible to make them. It is to be recalled, too, that the Army rates as used in this comparison, excluded certain groups of the Army which showed a decidedly lower death rate than those given herein, and that the total rate for the Army would have been somewhat lower had the complete monthly figures been available for comparison. The conclusion seems justified, then, that the disparity existing between the two rates was not more than is accounted for by the assembling of large numbers of country boys in camps under urban conditions.

^aEstimate made in manner described under "The effect of length of service in the Army," p. 90.

FACTORS TENDING TO MODIFY THE INCIDENCE AND MORTALITY OF THE RESPIRATORY DISEASES

In the following pages an attempt is made to glean from the available figures such facts as may show the effect of a number of varying factors on incidence and mortality of the respiratory group of diseases during the war. These varying factors have been considered in connection with different groups of the troops concerned, and the attempt finally is made to correlate the knowledge thus gained into a concrete conception of the pandemic as a whole; they include age, length of service in the Army, race, nativity, climate and weather, and housing.

AGE

As to the effect of age on the incidence of respiratory disease, we are able to present figures for deaths only, classified according to age.¹ These apply to the whole Army, wherever located, and include the deaths of officers as well as of enlisted men.

Deaths from influenza, bronchitis, and the pneumonias

Age group	Number of deaths	Per cent of total	Age group	Number of deaths	Per cent of total
Under 21 years	1,951	4.9	41-45 years	96	0.24
21-25 years	21,439	⁴53.9	46-64 years	75	.19
31-35 years	2,637	6.5	Total	39,827	100.00
^{36-40 years}	319	.8	Total	39,827	100.00

If the above percentages be applied to the age groups as determined in the calculation of the relative death rates for the Army and for civil life and to a total of 37,002 deaths, the total for these diseases reported for 1918, the following rates per thousand per annum may be deduced.

Age	Death rates per 1,000 per annum	Age	Death rate per 1,000 per annum
Under 21 years	8.77	36-40 years	7.51
21-25 years	14.30	41-45 years	5.88
26-30 years	14.20	46-64 years	7.25
31-35 years	11.10	46-64 years	7.25

These rates show nearly the same relations between the age groups as those published from civilian sources, though here the group 21-25 has a relatively higher rate than is usually given.

LENGTH OF SERVICE

It has long been known that men new to the military service are more liable to contract disease, especially disease of the classes under consideration, than are men long in the service. It is possible with the data at hand to present certain facts showing the degree of this increased susceptibility of the recruit.

The report cards of 34,446 deaths from influenza pneumonia show the length of service of the patient at the time of his admission to hospital.¹ Of

these deaths 9,847 or 28.56 per cent occurred in men of less than two months' service, 10,990 or 31.90 per cent in men of 2 to 4 months' service, 6,107, or 17.73 per cent in men of 4 to 6 months' service, 2,629, or 7.66 per cent in men of 6 to 8 months' service 1,663, or 4.83 per cent in men of 8 to 10 months' service, 1,198 or 3.48 per cent in men of 10 to 12 months' service, and 2,012 or 5.84 per cent in men of over 12 months' service. These relations are graphically shown in Chart XV. It is impossible accurately to estimate the proportion of men of each service group in the Army for the entire period of the war, inasmuch as the proportion was constantly changing with the passage of time; however, an attempt has been made to average the proportions found in different months. The resulting relative strengths probably approximate the distribution of men at the time of the greatest mortality in the fall of 1918. Based on these relative strengths, rates have been calculated showing the relative differences in death rates of the different length of service groups; these are given in Chart XVI and show the same general relations as Chart XV. The former gives a better relative idea of the importance of short service as a cause of death. It is seen that 60.46 per cent of all deaths occurred in soldiers of less than 4 months' service. It is to be noted also that the second bimonthly period shows a larger proportion than does the first. This is an unexpected finding and may be due to other factors than increased incidence among the men concerned.

It is obvious that if there had happened to be an unusually large proportion of men of the two to four months' group service in the Army at the time of the fall wave (1918) of influenza, and a correspondingly small number of men of less than two months' service, the number of deaths charged to the latter group would be relatively small in the total and those of the former would be too large. The same principles apply as to the estimation of death rates in commands of varying strength. As a matter of fact the number of two to four months' men was considerably larger in September and October, 1918, than was that of the less than two months' service men. This conclusion has been arrived at by studying the relations of the total enlisted strength of the Army from month to month. Thus, if the total strength was greater by 200,000 one month than in the month previous there must have been in that month 200,000 men of less than one month service. This gives us means of checking up on the figures already given. It is available only during the period of progressive increase in Army strength, and takes into account no losses from death or discharge, but it is believed that it is roughly satisfactory as a check. If the number of men of less than two months' service in each month from October, 1917, to October, 1918, inclusive, be compared with the total enlisted strength in the United States the same month, the percentage of recruits of that length of service may be obtained for each month. If, then, for each month the corresponding percentage of deaths among enlisted men in the United States be calculated and the results added the sum shows the number of deaths that should have occurred among these men had their rate been the same as that of all the other men. This forms a certain percentage of the total deaths in the whole Army, in the case of the less than two months' service men, 20.45 per cent. As a matter of record, as has been stated, these men actually had

CHART XV.-Percentage of total deaths from influenzal pneumonia during the war period occurring in each of the specified groups of length of service

CHART XVI.--The relative mortality rates per 1,000 strength from influenzal pneumonia during the war period in each of the specified groups of length of service

28.56 per cent of the total deaths, or one and four-tenths times their pro rata share. Applying the same methods to the two to four months' men, it is found that their pro rata share was 12.83 per cent of the deaths, while the records charge them with 31.9 per cent, or two and forty-eight hundredths times the former figure. It is impossible to carry calculation to older service groups inasmuch as after four months' service considerable numbers began to go abroad, and the calculation of strength becomes less accurate. However, it is seen that the figures conform to those of the former series in showing that the man of two to four months' service apparently showed a higher mortality from respiratory disease than did those of less than two months. As stated above, there were about 30 per cent more men of the longer service group in the service during the fall outbreak of influenza than there were of the less than two months' service men. This undoubtedly is the factor increasing their rate in the total.

The unfavorable effect on camp rates of the presence of a number of recruits is mentioned in a large number of reports of the fall epidemic (1918).¹¹ Camp Sherman, Ohio, had the highest mortality rate of any large camp; 46 per cent of its strength were classed as recruits. The rate of Camp Cody, N. Mex., was almost as high, with 69 per cent recruits; Camp Grant, Ill., 40 per cent; Camp Forrest, Ga., 55 per cent; Camp Devens, Mass., 30 per cent; Camp Custer, Mich., 33 per cent; Camp Greene, N. C., 50 per cent; Camp Syracuse, N. Y., 90 per cent. In the lower part of the mortality scale are found Camp Travis, Tex., with 4 per cent recruits; Las Casas, Porto Rico, with 5 per cent; Camp Sheridan, Ala., with 6 per cent; Camp Eustis, Va., with 7 per cent; and Camp McClellan, Ala., with 10 per cent. If the larger camps be divided into four groups according to mortality, we find the group with the highest mortality had 41 per cent recruits, the second 31 per cent, the third 22 per cent, and the fourth 16 per cent. This relation is the most clean cut of any found among the factors influencing the comparative rates of the camps.

Many specific instances of the high mortality of new men as compared to those of longer service are on record. At Camp Grant, Ill., for instance, the September inductants lost 4 per cent of their strength, while the loss for the balance of the camp was less than 2 per cent. Vaughan said:¹²"If recruits had not been sent to Grant in September, the camp mortality rate from the epidemic would have been 1.7 per cent; 16,000 recruits raised the rate to 2.6 per cent an increase of 53 per cent."

The report of the influenza commission working at Camp Pike, Ark., showed similar relations, as follows:¹³

	Strength	Percentage of influenza	Percentage of influenza cases having pneumonia
Seasoned men	28,782	15.5	11.1
Recruits	23,749	30.6	13.9

At Camp Funston, Kans., the relations were as follows:¹⁴

	Admissions	Deaths	Ratio
Seasoned men	33.8	1.39	1.0
New men	30.3	2.29	1.65

From Camp Lee, Va., the following report was made:¹⁴

	Months service
	Less than 1	1 to 3	Over 3
	Per cent	Per cent	Per cent
In camp	9.2	45.2	45.6
Epidemic deaths	30.1	46.2	23.7

At Camp Upton, N. Y., recruits were scattered among companies of the 2d Battalion in the proportion of three recruits to two older men, but no less than 167 recruits became sick before the first of the men who had been in camp prior to September 6.¹⁴ Eight recruits died to each man of longer service.

Analysis of 494 cases of pneumonia made by the laboratory officer at Camp Grant, Ill., showed the distribution of cases as follows:¹⁵ During the first month of service, 192; second month, 42; third month, 42.

A report from Camp Bowie, Tex., shows the large incidence of respiratory disease among recruits comprising a relatively small proportion of the strength of the camp, during the months of April, May, and June, 1918.¹⁶

At Camp Lewis, Wash., of 140 cases of pneumonia occurring during September, 1918, 107 were in men of less than one month's service and of 316 cases of influenza, the men of less than one month's service had 237.¹⁷

RACE

Of the various races shown in the general tabulations given herein, only the white and colored can be compared under corresponding conditions. The Philippine Scouts, the Hawaiians, and the Porto Ricans were so largely stationed in tropical or nearly tropical countries and in such comparatively small numbers that comparisons are of relatively little value. Suffice it to say that the Porto Ricans showed the highest admission rates for the total respiratory diseases of any group in the Army, while their death rate was well below the average.¹ The Philippine Scouts showed relatively low rates for admissions and deaths as compared to the Army as a whole, but had a higher admission rate and a very much higher death rate than those of the white enlisted men in the Philippine Islands.¹ The same general relations hold for the Hawaiians.¹ Owing to the effect of the favorable climate the mortality was not high but was higher in these native races than in whites living under the same conditions.

The comparison of the white and negro races, however, can be carried out under a great variety of conditions. The rates for admissions of the colored troops, as shown in the general tabulations, are higher than those for the whites

for every group except the troops in Hawaii. The death rates are higher in all cases, and usually very much higher. Thus the case fatality of the colored is seen to be much higher than was the case with the whites. More valuable deductions, however, may be made from the study of the monthly annual rates for admissions and deaths by races in the United States and Europe (see Tables 13-22). If the averages of the monthly annual rates in each group be taken for comparison it is seen that in the United States the total admission rate for respiratory diseases in the colored troops was slightly lower than that for the whites; however, this difference is due entirely to the small proportion of cases admitted with a primary diagnosis of influenza. The cases showing respiratory symptoms more markedly, the bronchitis and the pneumonias all showed a higher rate for the colored than for the white. It is possible that the colored recruit was slower on the average in reporting his illness, but inasmuch as the total figure is practically the same as that of the white the more probable explanation would seem to be that when attacked, the colored man averaged a much more severe case than did the white man. In Europe the colored troops showed an admission rate over double that of the whites for the same period and much higher than that of the colored troops in the United States. The highest admission rates then were those of the colored troops in Europe, followed in order by those of the colored troops in the United States, the white troops in the United States and the white troops in Europe. The rates of the latter are probably given too low in the tables quoted inasmuch as a considerable number of cases of respiratory disease were reported under the heading "Color not stated," somewhat over 24,000 in all, most of which occurred in Europe during the months of September and October, 1918. The comparison of the case fatalities of these cases and of the relative proportion of lobar to bronchopneumonia leads to the conclusion that most of these men were white. However, if all of these were added to the white admissions the relations would not be very materially changed and it has seemed better to deal with the figures as shown.

The relations between the death rates of the different groups is somewhat different. Here the highest average rate is that of the colored troops in the United States, 16.06 for the months covered by statistics for the colored in Europe. During the same period the colored troops in Europe showed a death rate of 11.78 per thousand per annum. For the 31 months for which we have figures for the white troops in Europe their rate was 3.72 as compared to 7.16 for the whites in the United States for the same months. Reverting to the effect of length of service then, it appears that while the seasoned white soldier shows to marked advantage both as to admission and death rates compared to the recruit, the effect of seasoning on the colored soldier is much less marked, and indeed under the conditions he was called upon to face in Europe his admission rate was higher than that of the relatively untrained men in this country. The seasoned colored soldier, however, did show a small gain in the matter of deaths, though even here the difference is by no means so marked as is found in the whites. The effect on case fatality of length of service as deduced from the figures for the troops in Europe and in the United States, is to increase the

figure for the white troops and to decrease it for the colored. It is to be remembered that these conclusions are drawn from the averages of monthly annual rates and not from the total group figures shown in general tabulations. For reasons already explained this method is believed to give the more reliable results.

While, as has been stated, the admission rates for the colored troops averaged slightly lower than those for the whites in this country for the period of the war, it is noticeable that this was due not only to a relatively small number of the milder cases but was influenced as well by the stage of evolution of the pandemic. Previous to the fatal wave of September and October, while the virus may be assumed to have been gathering virulence, the rates for the colored troops are shown to be decidedly higher than those for the whites. During the peak wave the whites showed more cases, and following this the two curves remain much closer together, the differences being hardly significant until a point well on in 1919 when the short-lived immunity conferred by the disease had begun to wear off. The colored rate then again began to rise above that of the whites.

In other words, the colored soldier is seen to have been relatively more susceptible to the infection in that he contracted it in larger proportions in the preliminary waves, thus acquiring an immunity that served to protect him against the more fatal wave which followed. A somewhat similar relation is to be noted between the white soldier of the North and of the South, as will be brought out later.

Another point in which interesting racial differences are shown by the figures is the relative proportion of bronchopneumonia to lobar pneumonia. It is well known that clinically the differentiation between these two types of disease is not always possible. Confluent types of lobular pneumonia may produce physical signs indistinguishable from those of lobar consolidation. Even post-mortem examinations may leave one in doubt. However, as was pointed out in an earlier paragraph, it was noted early in the war period that a large number of cases of pneumonia presented clinically and anatomically the characteristics of bronchopneumonia rather than those of lobar type. Granting the impossibility of accurate differentiation in many cases, still it must remain true that in the observation of thousands of cases the figures obtained are significant and variations in the proportion of one type of pneumonia to another between different groups of soldiers, or from month to month in the same group, may prove to be important in the study of the effect of race and length of service.

Table 12 shows the ratio of bronchopneumonia to lobar pneumonia for the different subdivisions of the Army for the period of the war. These figures, like those for case fatality, are independent of any strength estimations and so are strictly comparable. They are, however, summation figures for the entire 33 months and hence, since the great majority of the cases and deaths occurred in September and October, 1918, they more nearly represent the values for those months than an average. These figures show that for the entire Army the pneumonia ratio for officers was 1.05, for the white enlisted men 0.76, and for

the colored enlisted men 0.42. For the Army in the United States the corresponding figures were 0.84, 0.61, and 0.39, while for the Army in Europe, 1.30, 1.19, and 49. It is seen that the values of these ratios correspond inversely in a general way with the relative resistance of the various groups, the officers suffering least from the epidemic, the white enlisted men next, the colored enlisted men having the greatest losses. It is also seen that the corresponding groups show a higher ratio in Europe than in this country.

STATE OF NATIVITY

Figures are available showing the number of admissions and deaths from all the respiratory diseases according to the State of birth of the patient as given by him at the time of his admission into the hospital.¹From these it is possible to calculate directly the case fatality by States. The question of calculating rates of incidence and mortality from these figures is complicated by the fact that we have no knowledge of the number of men born in each State who served in the Army. Comparative rates have been published based on the total inducted strength from each State and probably in many cases this results satisfactorily. However, such rates take no account of foreign-born inductants, of whom there was a large number from some States, nor of the effect of migration from State to State. Certain of the Western States showing very low rates when treated in this way can be shown to have had in 1918 over three times as many men of military age as there were children 18 years younger in 1910. A similar effect on the rates is produced by the presence from a State of a large proportion of foreign-born inductants. Manifestly the excess is the result of immigration, and to base nativity rates on such an increased number of men results in a rate far too low. The reverse is true of States losing population by emigration. In order to obtain figures on which it might be possible to base comparisons of incidence, it seems best to prorate the inducted strength of the Army between the States in the same proportion as the States had boys under 10 years of age in 1900. This should result in a fair approximation to the number of men born in each State who served in the Army. Inasmuch as this method takes no account of the foreign-born soldiers the rates are of value only for comparisons between themselves. The method should allow us to state that the death rate in natives of one State was higher than in natives of another, but it does not permit of comparisons with other rates based on more complete data. Table 24 shows the number of admissions and deaths for total respiratory disease by native State for white enlisted men with rates calculated according to the method described, and the case fatalities. In the first column of Table 25 the States have been arranged in the order of incidence rates for white enlisted men beginning with the State having the lowest rate, while the second column shows the order with respect to mortality and the third case fatality. It is seen that there is a general correspondence between the two columns, though some differences are noted.

TABLE 24.-Relative admission and death rates, and case fatality for the respiratory group of diseases for white enlisted men, United States Army, by State of birth

State	Admissions	Rate	Deaths	Rate	Fatality
Alabama	18,348	312.2	545	9.29	2.97
Arizona	1,061	194.3	30	5.49	2.83
Arkansas	18,046	340.3	506	9.46	2.82
California	15,481	330.0	415	8.12	2.68
Colorado	6,032	270.0	261	11.70	4.33
Connecticut	8,349	242.0	311	9.02	3.73
Delaware	1,387	182.0	35	4.59	2.53
District of Columbia	1,644	183.2	67	7.46	4.08
Florida	7,319	453.0	282	17.45	3.85
Georgia	18,955	276.0	613	8.92	3.23
Idaho	1,964	240.0	67	8.19	3.41
Illinois	53,876	252.5	1,973	9.25	3.67
Indiana	29,301	273.0	992	9.26	3.38
Iowa	30,548	300.0	1,309	11.81	3.86
Kansas	23,451	349.2	937	13.95	3.99
Kentucky	18,208	189.0	920	9.55	5.05
Louisiana	14,800	339.5	596	13.67	4.03
Maine	6,871	281.5	292	11.97	4.25
Maryland	10,591	248.0	387	9.05	3.66
Massachusetts	24,870	236.5	928	8.82	3.73
Michigan	23,886	233.2	955	9.22	3.95
Minnesota	22,692	260.4	1,127	12.92	4.96
Mississippi	13,865	329.5	454	10.79	3.28
Missouri	39,803	277.5	1,424	9.93	3.58
Montana	2,650	262.4	90	8.91	3.40
Nebraska	15,390	297.2	606	11.70	3.94
Nevada	562	389.0	30	20.78	5.34
New Hampshire	3,684	256.0	143	9.92	3.88
New Jersey	16,502	210.5	689	8.75	4.16
New Mexico	2,324	220.3	100	9.48	4.30
New York	62,508	218.0	2,250	7.85	3.60
North Carolina	18,267	248.5	626	8.51	3.43
North Dakota	4,163	236.9	201	11.51	4.83
Ohio	40,021	236.1	1,772	10.47	4.43
Oklahoma	12,907	287.3	394	8.78	3.05
Oregon	5,543	335.0	169	10.22	3.05
Pennsylvania	60,435	219.0	2,361	8.55	3.92
Rhode Island	3,312	202.0	129	7.87	3.90
South Carolina	9,558	274.7	360	10.34	3.77
South Dakota	6,554	316.6	322	15.57	4.91
Tennessee	20,399	219.8	784	8.45	3.85
Texas	49,324	347.0	1,417	9.98	2.87
Utah	3,963	254.0	165	10.58	4.16
Vermont	3,281	260.0	188	14.88	5.73
Virginia	16,457	254.7	595	9.20	3.62
Washington	6,056	280.2	178	8.24	2.94
West Virginia	14,240	284.0	530	10.55	3.72
Wisconsin	26,208	264.3	1,184	11.95	4.52

Charts XVII, XVIII, and XIX are outline maps of the United States showing the relative rates for admissions, for deaths and the case fatality of respiratory disease, the rates calculated as described above. The death rates in particular and to a lesser extent the admission rates are fairly consistent, in that neighboring States of similar topography and similar density of population show similar rates. The extremely high rates of Nevada and the low rates of Arizona and of Delaware were based on relatively much smaller numbers of cases than was the case for most of the other States and possibly are consequently less reliable. The fact, however, that in all three of these States the figure for the case fatality falls in the same relative position tends to strengthen the admission and death rates.

TABLE 25.-Relative position of the States in respect of rates of natives for admissions, deaths, and case fatality from the respiratory group of diseases, white enlisted men, United States Army. Arranged in order from lowest to highest. The State showing the mean rate is italicized

	Admission rates	Death rates	Case fatality
1	Delaware	Delaware	Delaware
2	District of Columbia	Arizona	California
3	Kentucky	District of Columbia	Arkansas
4	Arizona	New York	Arizona
5	Rhode Island	Rhode Island	Texas
6	New Jersey	California	Washington
7	New York	Idaho	Alabama
8	Pennsylvania	Washington	Oklahoma
9	Tennessee	Tennessee	Oregon
10	New Mexico	North Carolina	Georgia
11	Michigan	Pennsylvania	Mississippi
12	Ohio	New Jersey	Indiana
13	Massachusetts	Oklahoma	Montana
14	North Dakota	Massachusetts	Idaho
15	Idaho	Montana	North Carolina
16	Wyoming	Georgia	Missouri
17	Connecticut	Connecticut	New York
18	Maryland	Maryland	Virginia
19	North Carolina	Virginia	Maryland
20	Illinois	Michigan	Illinois
21	Utah	Illinois	West Virginia
22	Virginia	Indiana	Connecticut
23	New Hampshire	Alabama	Massachusetts
24	Vermont	Arkansas	South Carolina
25	Minnesota	New Mexico	Florida
26	Montana	Kentucky	Tennessee
27	Wisconsin	New Hampshire	Iowa
28	Colorado	Missouri	New Hampshire
29	Indiana	Texas	Rhode Island
30	South Carolina	Oregon	Pennsylvania
31	Georgia	Ohio	Nebraska
32	Missouri	South Carolina	Michigan
33	Washington	West Virginia	Kansas
34	Maine	Utah	Louisiana
35	West Virginia	Mississippi	District of Columbia
36	Oklahoma	North Dakota	New Jersey
37	Nebraska	Colorado	Utah
38	Iowa	Nebraska	Maine
39	Alabama	Iowa	New Mexico
40	South Dakota	Wisconsin	Colorado
41	Mississippi	Maine	Ohio
42	California	Minnesota	Wisconsin
43	Oregon	Louisiana	North Dakota
44	Louisiana	Kansas	South Dakota
45	Arkansas	Wyoming	Minnesota
46	Texas	Vermont	Kentucky
47	Kansas	South Dakota	Nevada
48	Nevada	Florida	Vermont
49	Florida	Nevada	Wyoming

Consideration of these figures shows that for the period of the war there was no such preponderance of disease and death from respiratory disease among the natives of the Southern States as was brought out by Vaughan and Palmer for the pneumonias of the early months of the mobilization.¹⁸In all three columns (Table 25) the States showing rates above the average (indicated by italics) represent practically every general section of the country. It is not noticeable, moreover, that the States with the largest cities tend to have lower rates in both admission and mortality columns. The relation between the three series of data is interesting. The variability of the rates is not the same. Of

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CHART XVII.-The relative admission rates for the respiratory group of diseases for white enlisted men by State of birth

CHART XVIII.-The relative death rates from the respiratory group of diseases for white enlisted men by State of birth

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the three series the case fatality shows the least variation between the States with a coefficient of variation of 0.16, admission rates were most variable, coefficient 0.26 and the mortality rates stood between with a coefficient of 0.20. It is evident that the mortality rate must bear a direct relation to both the admission rate and the case fatality. For the series as a whole the coefficient of correlation between the rates for admissions and for mortality is + 0.67 ? 0.052. This is a high correlation especially when the variability of fatality rates is considered. It is thirteen times its probable error. Of special interest is the fact, however, that the coefficient calculated for the death rate and the case fatality figures is practically identical, namely, + 0.677 ? 0.052. The influence on the death rate of the two factors, admission rates and case fatalities, was then about the same. This would be easily understood if the admission rate and the case fatality could be referred to the same set of cases. That this is improbable, however, is shown by the fact that there is no correlation between these two series of rates. The coefficient of correlation between the admission rates and case fatalities is - 0.0962 ? 0.0955. The probable error is almost equal to the coefficient and the figure is entirely without significance. It appears then that two sets of causes, one affecting the admission rate or morbidity and the other the case fatality, were active in determining the differences in the death rates.

CHART XIX.-The case fatality rates (per cent) of the respiratory group of diseases for white enlisted men by State of birth

It is evident from study of the outline maps that neither Northern nor Southern States, Eastern nor Western, mountainous nor fiat, showed any preponderance of either admissions, deaths, or case fatality. It has been a commonplace observation for years that when numbers of individuals were gathered together from various places and held under common conditions,

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those from cities showed a lower rate of incidence from the sputum-borne diseases than did those from the rural districts. This has been noted especially for military concentrations of population but also has been shown to hold for students in universities and other similar assemblages. The reason for this presumably lies in the greater exposure to contact infection in the case of the city dweller, and the consequent development of an immunity specific or nonspecific which tends to protect against invasion by the germs of disease. A comparison is possible between the rates of the different States and the relative urbanity of their population. The United States Census Bureau classes as rural all communities of less than 10,000 inhabitants. This figure is probably too high for the purposes of this comparison, but is the only one available for use. Taking the percentage of rural inhabitants of each State as given by the 1910 census, which is the median census for the average age for the troops involved, and comparing the figures thus obtained with the rates for admission, death, and case fatality we find that between the admission rates for the war period and the percentage of rural population there is a definite positive correlation, the coefficient being + 0.362 + 0.083. This coefficient is large enough to be statistically significant and is over four times its probable error. It is not a high correlation but in view of the fact that the dividing line between rural and urban is probably too high for our purpose we are justified in drawing the conclusion that a rural population will show a higher morbidity from respiratory disease when inducted into the military service than will an urban one. A similar but smaller coefficient is found for the correlation between the rural population and the death rates. In this case the figure is + 0.311 ? 0.087. When the percentage of rural population is compared with the case fatality rates, however, all significant correlation disappears. The coefficient of correlation between these two series of data is + 0.16 ? 0.09; the low coefficient and the fact that it is not twice its probable error, deprive it of all significance. It would appear then that at least one of the factors entering into the admission and death rates is the relative urbanity of the population from which the troops are drawn, and that this variable affects the death rates through its effect on admissions and not by any demonstrable effect on case fatality.

If the relative immunity enjoyed by the city dweller when inducted into the military service be of specific character for the diseases under consideration it should follow that the rates for troops should vary inversely with those of home populations if both were exposed to great danger of infection. The death rates for influenza and pneumonia for the age group 20-29 have been calculated for the registration States of 1918. The specific rate for males of this group is not available from published figures of the Census Bureau, but it is believed that for comparative purposes the combined rate for both sexes will be equally significant. The correlation between these rates and those of natives of the corresponding States in the Army during the same period is low - 0.0639 ? 0.12 and it is less than its probable error. No inverse correlation is found to exist. The inverse correlation between the Army admission rate and the civil death rates for the registration States is slightly higher, - 0.2 ? 0.12, but still of such value as to be without statistical significance. That, however, infections of the character of those mentioned are more prevalent in urban communities is shown by the

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correlation between the death rates for the registration States and the percentage of rural population. The figure obtained in this case is - 0.44 ? 0.098. This is a coefficient comparable in significance with the positive figure obtained for natives of the various States when serving in the Army.

It may be that the actual fact will prove to be that the negative correlation between the Army and civil death rates should have shown a higher figure than that given, as two States, Vermont and Colorado, were responsible for nearly half of the plus values in the "xy" column of the computation. Both these States had death rates well above the average and in both instances this was due to an abnormally high case fatality rather than to a high morbidity. Inasmuch as the factors leading to high case fatality remain obscure it may well prove to be the case that these States are influenced strongly by some factor at present unknown which throws them out of alignment and destroys the correlation. However, there is no justification for throwing them out of consideration at present, and the only conclusion justified by these figures is that there is practically no inverse correlation between the rates of the home populations and the natives of corresponding States when serving in the Army. From this would follow that the civil population had previously developed no immunity to influenza and influenza pneumonias, in proportion to its urbanity, and that consequently such relative immunity as was shown by the relatively urban soldiers was not of a specific nature. If the civil rates for a nonepidemic period, 1913, 1914, and 1915 (same age group), are compared with the Army rates in 1918, similar results appear. The coefficient of correlation here is - 0.167 ? 0.134. While the coefficient has the minus sign its size and its relation to its probable error are not such as to give it statistical significance. It seems probable, therefore, that the relative immunity enjoyed by the city dweller in the Army was the result of nonspecific rather than specific factors. This is of course borne out by the fact that the city dweller also shows a relative immunity to such diseases as epidemic meningitis which are not known to be at any time so prevalent in a population as to induce any appreciable specific immunity.

Following the suggestion in the work of Pearl,¹⁹who showed that there was a definite relation between the explosiveness and fatality of the influenza epidemic in cities and the total death rate and especially the rates for pulmonary tuberculosis, organic heart disease, and nephritis, the attempt has been made to correlate the Army death rate with the rate for the States in this regard. The death rates for the registration States in 1913 for tuberculosis, organic heart disease, and nephritis (age 20-29) were calculated and coefficients computed. That for the correlation between these rates and the case fatalities in the Army was + 0.179 ? 0.11, a coefficient without significance statistically. The same is true of the correlation between the tuberculosis, organic heart disease, and nephritis rates of the civil population and the Army admission rates. The coefficient here is - 0.20 ? 0.11. This failure of correlation may be due to the fact that the element of the population whose organic weakness leads to this relation between the rates in civil life was almost entirely weeded out of the Army by the examining boards. Further the figures are not complete as the registration States of 1913 represent less than half of the States of the Union.

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Certain other factors very probably entered into the production of the variations of rates between States. There may have been differences between the States in the matter of relative number of recently inducted men at the time of the fall epidemic of influenza, which was responsible for the greatest number of admissions and deaths. An attempt to determine this factor from the records fails to reveal any significant differences. Another factor, however, also difficult or impossible of demonstration, undoubtedly had its effect. This was the fact that the recently inducted men of some States were assembled at camps which showed much higher mortality during the pandemic wave of the disease than was the case for others. The comparison between the camps is brought out elsewhere. Inasmuch as the greater part of the mortality was among the recently inducted, this difference between the States is one that must have had its effect. It is probably impossible to evaluate it accurately, but it undoubtedly was one of the factors that tended to throw certain States off in the various correlations that have been recorded.

For the period of the war, then, it can not be said that the inhabitants of any one section of the country showed a marked advantage over those of any other in the matter of morbidity or mortality from influenza and pneumonia. The evidence in the figures indicates that there are two sets of causes acting separately to produce the mortality rates, one acting through the admission rates and the other through the case fatalities. One of the causes tending to increase the morbidity rate is the relative proportion of rural inhabitants in the States from which the troops come. No general cause has been discovered to account for the variations in case fatality between the States. It has proved impossible to show any correlation between the rates for tuberculosis, organic heart disease, and nephritis in the States and either the admission or case fatality rates of the corresponding troops. The fact that there was no significant negative correlation between the Army rates and civil rates for the same State during the year of the influenza epidemic is interpreted to indicate that the civil population had not acquired any specific immunity to influenza in proportion to its urbanity and that the relative immunity shown by men from more urban States was of a nonspecific character.

During the earlier months of the mobilization the relation of the nativity to morbidity from respiratory disease was much more marked. In the exhaustive study of the subject made by Vaughan and Palmer¹⁸ it was possible to show that the total mortality and, in particular, the admission and death rates for the pneumonias were much higher in camps that drew their troops from the South Atlantic and Gulf States. They show the camps located not in their geographical position but placed in the center of the area from which they drew their troops; also that camps drawing their troops from Florida, Georgia, Alabama, Mississippi, Louisiana, and Arkansas exhibited rates much higher than the average. The camps showing the lowest rates drew their men from the Northeastern, North Central, Northwestern, and Pacific States. Of the Northern States, those sending troops to Camp Bowie and Dodge showed the highest rate. These States, Minnesota, North and South Dakota, Nebraska, and Iowa, are all shown to be above the average for mortality for the period of the war in Table 26. The Southern States, however, show no such marked agree-

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ment between the figures for the war period and those of the first six months of the mobilization. The figures given by Vaughan and Palmer¹⁸ show enormous disparities between the rates of these States and the others; however, these figures appear to be without value for comparative purposes, inasmuch as no allowances have been made for increased susceptibility of the negro troops at this time.

During this period the liability of the negro troops to contract lobar pneumonia was eight and one-half times that of the white troops.²⁰ In Alabama there were of military age in 1918 approximately 58 per cent of whites and 42 per cent colored.²¹ If these percentages contracted pneumonia in the proportion given above, a simple calculation shows that the white, 58 per cent, furnished only 14 per cent of the pneumonia and the colored, 42 per cent, furnished 86 per cent. There are no data available on which relative nativity rates for the white troops can be calculated for the early months. The nativity tables in the Annual Report of the Surgeon General, United States Army, for the year 1918, do not separate the white and colored. However, the conclusion seems justified from the study of the camps mentioned above that the Southern States showed a much higher morbidity and mortality from the respiratory diseases during the last three months of 1917 and for the first three months of 1918 than did the other States and, further, a relatively much higher rate than the Southern States themselves showed for the whole war period. This relation of their rates will be discussed later and a tentative explanation advanced.

If the rates for the war period for groups of States are calculated the results are found to be as follows:

Group	Relative admission rate	Relative death rate	Fatality
New England	243	9.61	3.96
Middle Atlantic	217	8.26	3.60
East North Central	241	9.95	3.97
West North Central	291	12.10	4.50
Mountain and Pacific	279	9.38	3.38
East South Central and South Atlantic	257	9.44	3.66
West South Central	335	10.30	3.07

These relations are shown graphically in Chart XX, which also shows a line indicating the percentage of rural population in each group of States. This chart shows, as did the correlation, that there is some relation between the percentage of rural population and the admission rates, but none between the percentage rurality and the case fatality. Chart XXI compares the death rates for the groups of States as calculated for the war period with the rates for the same sections of the country given by Vaughan and Palmer¹⁸ for the early months of the mobilization. It is seen that the greatly higher rates for the rural States shown in the early months did not hold for the whole war period. It must follow, then, that in the later months when the virulence of the influenza epidemic was at its height, the relative rate for the more urban States exceeded that for the rural ones.

The figures for the colored troops as regards the effect of nativity on incidence and fatality from respiratory disease are given in Table 26. States furnishing an insignificant number of colored troops are not included in this table.

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The States included furnished over 99 per cent of the colored troops in the service. Owing to the well-known difficulty in obtaining accurate information for record from members of this race, it is believed that the figures are probably not as satisfactory as those for the whites. A comparison of the death rates by States for the two races, however, shows that in general the same relative positions hold. If the correlation between the death rates by States for the two races be calculated, a coefficient of + 0.613 ? 0.086 is obtained. Apparently the same conditions that lead to a high mortality in the whites from a certain State when serving in the Army also tend to produce a high mortality among its colored soldiers.

CHART XX.-Relative admission and death rates and case fatalities for the respiratory group of diseases in the various groups of States, April, 1917, to December, 1919. For facility of comparison, the different rates are drawn to a different scale. The percentage of rural population for each group is also shown

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TABLE 26.-Relative admission and death rates and case fatality for the respiratory group of diseases for colored enlisted men by State of birth. April, 1917, to December, 1919. Only those States showing one thousand or more admissions are included

State	Admissions	Relative admission rate	Deaths	Relative death rate	Fatality
Alabama	7,751	234	466	15.0	6.0
Arkansas	4,325	269	229	14.3	5.3
Florida	3,547	391	213	23.4	6.0
Georgia	11,602	267	718	16.4	6.2
Kentucky	2,250	268	114	13.1	5.07
Louisiana	8,275	328	615	24.6	7.44
Maryland	2,019	276	106	14.5	5.25
Mississippi	7,584	200	417	11.0	5.5
Missouri	1,051	242	72	16.6	6.85
North Carolina	6,121	230	328	12.0	5.36
Pennsylvania	1,017	235	39	8.7	3.84
South Carolina	10,530	279	638	18.1	6.05
Tennessee	4,467	262	307	18.0	3.87
Texas	9,299	364	338	13.3	3.64
Virginia	5,865	240	348	14.0	5.93

CHART XXI.-Relative death rates from the respiratory group of diseases by groups of States for the war period with the figures given by Vaughan and Palmer for the early months of the mobilization. The differences shown in the early months are largely obliterated in the longer period.

CLIMATE AND WEATHER

Tables 10, 11, and 12 show that the effect of the influenza epidemic in increasing mortality was felt much less in troops stationed in tropical or subtropical climates than in the temperate regions. The effect on incidence of influenza of the warmer weather was much less than upon the death rate. Comparable groups of troops showed nearly if not quite as high an incidence

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rate in tropical stations as in the United States, but the case fatality of the epidemic was far lower. The case of the white troops in Panama, with an incidence rate of 166.18 per 1,000 (higher than for the American Expeditionary Forces) and a case fatality of only 0.27 per cent, is a good example. The experience of the Porto Rican troops at Camp Las Casas is also to the point. During the October epidemic 16 per cent of its strength contracted influenza, and but 0.52 per cent died. These troops when transported to the United States showed at least as high a fatality as the whites with whom they served. The experience of the white troops in the Philippine Islands and in Hawaii was similar. In these comparisons, however, the element of length of service is difficult to eliminate, as in general the troops at these distant stations were more permanent and averaged longer service than the troops in the training camps in the United States.

The effect of climate on the incidence and mortality of troops in the United States, however, is fairly well seen. Of camps of over 5,000 strength, 16 were situated in the North and 24 in the South. The incidence of influenza as compared to the average was as follows:²²

		Above average	Below average
16 northern camps	10	6
24 southern camps	9	15

In camps of a strength between 1,000 and 5,000 the relations were as shown below:

		Above average	Below average
24 northern camps	10	14
20 southern camps	11	9

In stations of less than 1,000 strength the following relation held:

		Above average	Below average
37 northern camps	19	18
31 southern camps	15	16

It is seen from the above tabulations that in the larger, northern camps there was a tendency to a higher incidence of influenza, but that this relation did not hold in the smaller camps. The greatly larger size of the camps of the first group thus impresses itself on the totals, and as a whole the incidence of influenza was higher for camps in the northern part of the country. There were, however, notable exceptions even among the larger camps. Camp Beauregard, La.; Camp Bowie, Tex.; and Camp Cody, N. Mex., were the three camps having the highest percentage incidence of influenza, and all three were southern camps. The same variable that interferes in so many

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comparisons, and which is so difficult of evaluation, seems to apply here. What was the relative proportion of recruits in these camps? We know that as a class the larger camps contained almost all of the short-service men. The men at the smaller stations were usually selected for special service, and the personnel of these stations had passed through the larger camps. The conclusion that seems justified by the reported facts is that there is very little difference between the northern and southern camps as far as the incidence of influenza goes. What difference there is appears to be confined to the larger camps and is perhaps due to a greater proportion of recruits in the northern camps, a factor that can not be estimated, or perhaps to the fact that the recruit was more susceptible to influenza in a northern climate.

When the mortality rates are considered, however, the advantage of the southern camps becomes evident. The corresponding figures for mortality follow:

		Above average	Below average
Large camps:^a

16 northern camps	13	3
24 southern camps	7	17
Medium camps:

23 northern camps	13	10
21 southern camps	9	12
Small camps:

38 northern camps	27	11
30 southern camps	7	23

^aThe discrepancies between the number of camps listed in the incidence and mortality tables is due to the fact that in each case only camps reporting complete figures are included. Hence the camps are not absolutely the same in the two sets of tables. The comparative value of the figures is not affected thereby.

From these figures it becomes evident that the mortality from the influenza epidemic was decidedly greater in the North than in the South. Inasmuch as the mortality has been shown to have been entirely due to the complicating pneumonias, we may say that while a soldier in a southern camp was just about as likely to contract influenza during the epidemic as his comrade in the North, his chances of complicating pneumonia and of death were very much less.

This corresponds very well with the reports from tropical stations, and it is possible to infer the generalization that while troops in warmer climates have about as much influenza during an epidemic as those in colder climates, their mortality from complicating pneumonia may be expected to be much less.

During the earlier months of the mobilization the camps that suffered most from pneumonia have been shown to have been without exception southern camps. That this was not due, however, to their location is shown by the fact that other camps, often only a few hours' travel distant, showed low pneumonia rates. The mortality in these camps, such as Camp Pike, Ark.; Camp Wheeler, Ga.; and Camp Travis, Tex., was due to the special susceptibility of their personnel; the controlling factors have been studied under the heading of influence of nativity.

The effect of weather on the epidemic is one that is difficult to estimate. Expressions of opinion by individual officers, sometimes even in the same camp, are at variance. In general it may be said that the weather at the time the great epidemic first put in its appearance in September, 1918, was fine

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throughout the country. The month was somewhat cooler than the average for some years past, but even a cool September would not be expected to be cold enough to cause hardship. In some camps rainy weather prevailed during the epidemic wave; in others fair weather was reported. Of 111 stations reporting, 89 characterized the prevailing weather at the time of the outbreak as mild, 22 as severe. Of those reporting mild weather, 43 showed a mortality above the average of their respective groups, 46 below average. Of those reporting severe weather, 12 showed a mortality above average and 10 below. It is evident that weather conditions were favorable, as a rule, during the epidemic and that the mortality was little if at all influenced by severe weather when it occurred.

HOUSING CONDITIONS

It is possible to study the effect of housing conditions on the course of the epidemic from two points of view: First, the space assigned to each man in the barracks, the effect of crowding; second, the effect of the type of quarters, whether tents or barracks. It is difficult by any ordinary methods to obtain figures for either of these variables that are clean-cut and are not influenced by other factors known to complicate the situation. It is impossible to estimate with any degree of accuracy the proportion of short-service men in the different camps and as has been shown a considerable difference in this respect would introduce a factor in the comparison that would materially alter results. The same may be said of the geographical situation if comparisons are based on mortality though this is less a factor when incidence rates are compared. However, certain conclusions may be justified and, accordingly, the results of the study are given briefly.

It is evident from the study of the death rates, reported by the Census Bureau, that, in general, cities suffered from the influenza epidemic more than did the rural communities. This accords with the general experience in pneumonia mortality over a number of years. Therefore, it is to be expected that concentration of population, affording increased facilities for the transmission of the virus, would increase the incidence of the disease. Vaughan has divided the various camps existing during the epidemic into three groups-those over 5,000 men, those between 1,000 and 5,000, and those under 1,000.²³ In general it may be said that the larger the command the greater the chance for dissemination of infection and the greater the probability of crowded conditions. Chart XXII shows the result of his study in this respect. It is seen that while the size of the camp shows little effect on the incidence of influenza, the proportion of cases developing pneumonia and the number of deaths are greatly affected, the smaller camps showing a much smaller proportion of complications. A more detailed study by comparing not only the arithmetic means of the camp rates but the medians and modes as well shows that the incidence rate of influenza also was decidedly higher in the larger camps. How much of this is due to differences in physical surroundings and how much to the well recognized fact that the men in the smaller camps averaged much longer service than those in the larger, can not definitely be said.

Data as to the degree of crowding in the various large camps during the epidemic are difficult or impossible to obtain in reliable comparable form.

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Figures that are obtainable seem to indicate little if any difference in incidence or mortality between those reporting crowded conditions and those showing the reverse. Such results, however, are open to the disturbing influence of the other variable factors mentioned above. It would seem that the influence of this factor is best estimated by comparisons made between different organizations in the same camp whose surroundings are substantially the same and which show practically the same proportion of recruits. A study of this character was made at Camp Humphreys, Va., during the fall epidemic.²⁴The organizations reported on are divided into two groups-those in existence some time and those newly formed. The results are tabulated as follows:

Organization	Floor space per man	Sick with influenza	Organization	Floor space per man	Sick with influenza
Older organizations:	Square feet	Per cent	Newer organizations (sapper regiments):	Square feet	Per cent
7th Regiment	46	26.7	217th Regiment	55	24.5
3d Regiment	46	28.6	218th Regiment	59	20.8
5th Regiment	47	16.0	219th Regiment	68	19.3
2d Regiment	50	9.1	220th Regiment	103	13.6
Engineer Officer's Training School	70	8.8	215th Regiment	114	9.3
4th Regiment	75	7.4

6th Regiment	78.5	2.5

The inverse correlation between the amount of floor space per man and the percentage of infection in comparable organizations is striking. While this is the only detailed study of this kind of which record is available, suggestions of a similar relation are found in the reports from a number of camps. It is believed that such reports, dealing with otherwise comparable groups, are of more value than the massed figures from a number of camps. It seems fair to conclude, therefore, that there is to be expected a definite relation between the degree of crowding and the amount of respiratory infection.

There appears to have been little difference in influenza incidence between the tent camps and the barracks camps. If the mortality rates for all the tent camps are compared with those of all the barracks camps, there is a decided difference in mortality in favor of the former. However, as with one exception all the tent camps were situated in the South, while the barracks camps were about equally divided, it is seen that the climatic difference elsewhere discussed interferes with the comparison. If all the northern camps be excluded from comparison it is seen that there is little or no difference in mortality for the two groups. Camps with relatively high and relatively low mortality are found in both classifications.

MODE OF TRANSMISSION

The actual mechanics of the mode of transmission of the virus of influenza is a point over which argument has taken place. There is to-day substantial agreement that the disease is transmitted from individual to individual, rather than by aerial convection, although the latter hypothesis has found many proponents in the past. The known facts of the matter may all be explained without recourse to the theory of spread by the air.

The generally accepted idea of the method of spread of this and similar diseases has been expressed in the general term applied to them, that of

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"sputum-borne diseases." Most of the preventive work directed against influenza and the pneumonias has been based on this idea of the principal method of spread. The obvious fact that infective material is constantly sprayed into the air by the coughing patient, from which it is equally readily inhaled by those near by, has tended to render us oblivious of other possibilities perhaps as important. The r?le of the hand in the spread of these diseases has been emphasized, particularly by Lynch and Cumming,²⁵and the importance of "hand-to-mouth" routes in disseminating infection has received much study. It can not be said that any agreement has been arrived at as to which method is of the greater importance in spreading disease. It is entirely probable that both methods play their part in the process.

CHART XXII

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That the number of new cases depends to a great extent on the closeness of contact between infected and noninfected individuals is shown in a number of ways. Though figures as to the crowding of camps during the epidemic are inconclusive, special studies made of the relation between floor space and influenza incidence in different units of the same camp, as shown above, indicate that there is a definite relation between crowding and the spread of the respiratory disease. The influence of varying degrees of closeness of contact is shown by figures reported from Camp Custer, Mich., during the fall epidemic (1918).²⁶

Influence of contact on incidence and mortality, Camp Custer, September-October, 1918

		Percentage contracting influenza	Percentage contracting pneumonia	Deaths
Total camp	23.4	5.4	1.7
Medical and dental officers	25.6	5.5	1.2
Other officers	6.6	1.4	0.0
Ward men in base hospital	11.7	2.5	0.8
Other Medical Department men in hospital	12.5	2.6	1.5
Army nurses	30.6	10.4	1.4
Civilian nurses	36.4	6.6	0.8

This tabulation shows plainly the increased incidence in groups coming in closest contact with infected individuals.

Vaughan (Warren) showed a similar relation among civilians in Boston.²⁷ His figures showed that "sleeping contact" was over twice as apt to result in infection as the less intimate forms of contact in the family. These figures seem to have some significance in the question of the relative importance of the hand and droplet in transmission. Apparently the latter method is more concerned in sleeping contact than the former. Lynch and Cumming²⁵ maintained that the indirect transmission of infection from hand to hand by means of infected dishwater in the dipping method of washing mess kits was the major route of transmission. The figures presented in their report taken from organizations of the port of embarkation, Newport News, Va., support their contention satisfactorily. They have also shown by experimental methods that the route they suggest is a feasible one. Their conclusions have been criticized, however, on the ground that other factors known to be concerned in the incidence of the disease, such as length of service, crowding, etc., were not eliminated in making their comparisons. Other officers have failed to show a similar relation between their organizations.²⁸At Camp Jackson, S. C., a group of organizations, carefully using boiling water in the washing of mess kits but composed of men of short service, showed a very much higher incidence rate during the fall outbreak than did another organization of much longer service that took no special care to effect thorough sterilization of dishes. Both used the dipping method.²⁹Here apparently the element of length of service was far more important in determining morbidity than was the sterilization of the mess kits.

Special bacteriologic experiments conducted at Camp Meade, Md., showed that, even when relatively cool, the soap in the dishwater was sufficient to kill the usual organisms found in respiratory infections, such as the pneumococci

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and streptococci.³⁰ These experiments, however, did show the possibility of the transference of organisms of the colon group by this means. As far as the influenza virus is concerned, the results are entirely inconclusive, as the exciting agent of this disease is not yet definitely recognized.

The consensus of opinion, based on the experience of the epidemic, would seem to be that while the r?le of the hand in the direct and indirect transmission of respiratory disease should receive careful study and fullest consideration, the claim that this represents the major avenue of spread of these diseases can hardly be regarded as proved, and attention to this possible means of travel of the virus should not be allowed to draw attention from methods of prevention based on the more usually accepted theories of the method of transmission.

In spite of all evidence pointing to the importance of contact, attempts to transmit the disease experimentally under controlled conditions have uniformly failed. The United States Public Health Service sponsored two experiments of this character during the fall outbreak in 1918.³¹ One experiment was carried on in Boston, with 100 volunteers from the Navy, of the most susceptible age. None were known to have had influenza previously. These men were treated with influenza bacilli, with nasopharyngeal secretions, with and without filtration, by intranasal sprays, and by direct swabbing from patient to volunteer. The attempt was made to induce the disease by the injection of citrated blood from patients and the injection of filtrates of nasopharyngeal secretions. Finally these men were exposed to the most intimate personal contact with patients in wards, all with the complete failure to produce the disease. A similar experiment was conducted with the same result in San Francisco. The explanation of this result is lacking. Either the proper method of transmitting the disease was not used, which seems very unlikely in view of the diversity of methods employed, or the volunteers themselves were immune to the disease either naturally or through previous infection in spite of their negative history. This latter hypothesis is hardly satisfactory though seemingly more probable than the former.

It can only be said then that the experience of the war has confirmed our previous belief that influenza is carried by infected persons and not for any distance through the air. The exact means by which the virus is transmitted from person to person, as well as the usual portal of entry in each case, remains unknown. While other means of transmission can not be excluded, and doubtless play their part, the known facts are not inconsistent with the generally accepted idea that the secretions of the respiratory tract expelled into the air by the act of coughing and inhaled by susceptible persons in the immediate neighborhood constitute the most important route.

As to the duration of the period of infectivity of the individual case, no reliable deductions may be made. The negative attempts at transmission experiments already quoted suggest the possibility that the infective period is very short, possibly even limited to the period of incubation or invasion. This idea is also supported by the observation that different methods adopted by different commands in the handling of their influenza cases apparently produced little effect on the incidence of the disease. Some camps attempted

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the immediate hospitalization and isolation of all suspicious cases, others reserved their hospitals for the seriously ill only, leaving the lighter cases for treatment in regimental infirmaries or in quarters where opportunities for transmission to the uninfected would appear to be much more numerous. It is not possible to show that this latter method of handling the situation resulted in any increase in the relative number of cases. The possible explanation may lie in a very brief period of infectivity, limited to the period of invasion or the earliest hours of the demonstrable presence of the active disease.

PREVENTION

With the preceding studies in mind, it appears evident that the prevention of the fatal pneumonias that attack armies may be approached from two points of view. First, the classical one that pneumonia is a primary disease directly due to the dissemination of the various organisms to which pulmonary inflammations may be attributed, aided by such well-recognized predisposing causes as chilling and exhaustion. Second, the point of view developed in the preceding pages which seems to show that during the period of an influenza cycle at least the great majority of pneumonia cases bear a direct relation to the prevalence of the so-called common respiratory diseases which during such a period appear to be definitely influenzal in origin. The second point of view is evidently the one applicable to the period of the World War. The figures cited and the relations shown make the conclusion inevitable that had it been possible to exclude the action of the influenza virus from the Army, the pneumonia mortality would have been so far less than it actually was as to have been of very little importance in the death records of the war. This statement holds for the months preceding and following the great fall outbreak as well as for that period.

While it may be shown that certain measures tend to diminish the proportion of influenza cases complicated by pneumonia, the prevention, or limitation, of the number of such cases remains the fundamental problem in the prevention of such pneumonia as was seen during the war period. This problem has not been solved. Before proceeding to a short r?sum? of the various means by which the prevention of influenza was attempted during the war, and the attempt to assign to them their relative value, it may be admitted that as far as the experience of the last pandemic goes, no practicable preventive measures have shown themselves to be of decisive value. Such measures as have shown some value appear to serve the purpose mainly of delaying the spread of the infection, of lessening the explosiveness of the outbreak. In a military camp this is an accomplishment of no small value inasmuch as it serves to reduce greatly the daily number of admissions during an outbreak, and correspondingly to lessen the strain on hospital facilities and personnel, with the result of giving to the individual patient the possibility of better care and increased chance of recovery. Preventive measures accordingly should be judged by the measure of their ability to prolong an outbreak by the diminution of its explosiveness, as well as by their ability to lessen the percentage of persons attacked.

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MEASURES DESIGNED TO PREVENT THE ENTRANCE AND SPREAD OF INFECTION IN A COMMAND

QUARANTINE

Absolute quarantine has been shown definitely to exclude influenza. The experience of Fort St. Philip, on the Mississippi River below New Orleans, is a case in point. This post was able to maintain an effective isolation and entirely escaped infection during the fall wave of the disease.³²The San Francisco naval training station, situated on an island, carried 4,950 men through the height of the epidemic without a case.³³However, very few stations are so situated as to be able to maintain perfect isolation, certainly none of the size of the great war training camps. Certain camp commanders, recognizing the futility of attempting quarantine in the face of the necessary supply problem, troop movements, etc., made no attempt to enforce isolation. Others restricted intercourse between their commands and adjacent communities in so far as it was possible to do so. There does not appear to be any significant difference between the two groups of camps thus divided. Vaughan's studies showed the following relations in this respect.²²

		Influenza incidence			Influenza incidence
	Above average	Below average	Above average		Below average
Large camps:			Small camps:

34 quarantined	17	17	38 quarantine	19	19
6 not quarantined	3	3	8 not quarantined	3	5
Medium camps:			Total:

27 quarantined	10	17	99 quarantined	46	53
7 not quarantined	4	3	21 not quarantined	10	11

It is seen that the totals show that 53 of 99 quarantined camps showed an incidence below the average, while 11 of 21 unquarantined camps were also below average in this respect. The respective percentages were 53.5 and 52.4. There is here no significant difference leading us to believe that such quarantine regulations as proved practicable during the war are of any value in reducing the total incidence of disease. In the six large camps that did not attempt to enforce quarantine, the duration of the epidemic was as follows: Camp Sheridan, Ala., 3 weeks; Camp Jackson, S. C., and Camp Taylor, Ky., each 4 weeks; Camp Forrest, Ga., 5 weeks; Camp Humphries, Va., 6 weeks, and Camp Logan, Tex., 7 weeks, an average of 4.8 weeks as compared with 4.9 weeks for the 36 large camps for which information is available.³⁴There is no evidence here that quarantine availed to prolong the outbreak and thus distribute the cases over a longer period. To be of avail in excluding influenza, quarantine must more nearly approach perfection than proved practicable in the large camps of the war period. The experience of certain civilian institutions too, from which the disease was excluded by quarantine until the subsidence of the outbreak, only to have it appear promptly as soon as restrictions were removed, seems to indicate the futility of general quarantine as a military measure. That certain camps, where great stress was laid upon quarantine, had little influenza is true. The experience of Camp Wheeler, Ga., has been quoted in

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support of this measure.²²The Camp Wheeler report states that quarantine was effective in limiting the disease almost entirely to recruits who brought it with them. The personnel of Camp Wheeler, other than the new men, was very small and was composed almost entirely of men who had passed through the relatively severe spring outbreak in that camp, when indeed the influenza rates were higher than they were in the fall. The fact that these men were all of long service and had passed through one well defined outbreak less than six months before is a more probable explanation of their immunity than the institution of quarantine. On the other hand, Camp Humphries, Va., which instituted no quarantine, had a lower incidence of the disease than did Camp Wheeler.

The prohibition of mingling of commands within the camp has received rather general indorsement. Its value appears to lie in the reduction of the explosiveness of the outbreak. The following extract from a report from Camp Lee, Va., illustrates this point:³⁵

It is very doubtful whether any measures taken reduced the incidence of the disease. The quarantine seemed to have no ultimate effect, but did delay the appearance of the disease in the organizations so isolated. For instance, the veterinary training school of about 3,800 men established a most rigorous quarantine and all members of the command had their noses and throats sprayed daily with argyrol, consequently they had very few cases, until October 5, when the epidemic reached a sudden peak and then rapidly declined, they being practically free from the disease in one week thereafter. Therefore, it would seem that the only benefit from the measures taken was that this camp was not overwhelmed at any one time by the number of sick. The disease was spread over five or six weeks, allowing better care of the sick.

It would appear, therefore, that experience during the World War indicated that while quarantine regulations are powerless to protect a large command from infection during epidemic outbreaks of influenza, the restriction of intercourse between the different organizations of a command may be of great value in prolonging the outbreak, thus permitting better care of the sick.

Of the 34 large camps that instituted quarantine, 23 conducted a special medical examination of all new men entering camp with a view to the detection of infected individuals and their prompt separation from the uninfected; 19 of the latter also placed the new men in a detention camp, and 8, in addition to the above measures, gave all men joining a prophylactic spray.³⁴These 8 camps showed an influenza incidence, during the epidemic, of 22 per cent as compared to the 23 per cent average incidence of the large camps as a whole. It is not evident that these measures availed materially in preventing the entrance of infection. Although no figures are available to support the contention, it would appear that physical examination of new arrivals in a camp should tend to diminish the explosiveness of an outbreak, as the principles are similar to those that govern in the matter of interorganization quarantine. The objection found to measures of this character, however, was the practical one that in times of epidemic the number of officers available for duties of this character is very small, and the rush of work such as, almost inevitably, to result in hasty, more or less perfunctory examination. Such examinations are of little value and take medical officers away from other, possibly more important duties.

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MEDICAL INSPECTION

Medical inspection, carefully performed, is of unquestioned value in securing early treatment of the sick. It is well known that during an epidemic men are often slow to report for treatment and frequently persist in going about their duties for hours or days when actually ill. The better the morale of the troops, especially in the face of the enemy, the greater the danger of this taking place. It is mentioned in reports from the American Expeditionary Forces that men kept their places in the ranks until forced to fall out, often with fully developed pneumonia. Daily, or twice daily, medical inspection of the men, taking temperatures in suspicious cases, serves to detect such cases early and to reduce the likelihood of serious complications. There is, however, nothing to show that the institution of this measure materially reduced the incidence of influenza in the camps employing it. Under practical conditions the difficulty of devoting to this measure the time necessary to render it effective makes its satisfactory application almost impossible. Of the 78 camps employing daily inspection of the troops, half were above the average in incidence and half below. The same results were obtained in the 30 camps that did not institute it.²²

USE OF THE MASK

The value of face masks worn by the whole of a command has been the subject of much argument. In the first place, if we hold with those who maintain that the main route of infection is "hand to mouth" most of the theoretical value appears to disappear, though it may limit to some extent the number of times the hand visits the mouth. It appears to be generally conceded that the use of the mask by attendants on the sick, exposed constantly to infection, is of value. That the use of the mask was universal among hospital attendants is a fact. It is equally true, as shown by the Camp Custer figures given above, that such attendants, especially nurses, were attacked in a much higher percentage than the average. They were of course constantly exposed and were undergoing severe strain in the performance of their duties. Under these circumstances judgment as to the value of the mask becomes difficult. Reports from the Durand Hospital in Chicago indicated great value in protecting attendants and in preventing cross infection among patients.³⁶

It appears that masks, to be effective, should be of a certain definite thickness of material and that there are differences in the value of different materials used for their construction. Reports from Camp Grant, Ill., in the earlier months of the mobilization, while confirming the impression that cross infection may be limited by this means, have shown that a certain critical thickness of gauze must be used to prevent the passage of bacteria.³⁷Too great a thickness was found to result in an uncomfortable mask that in many instances did not allow the free passage of the breath, necessitating the passage of the latter around the mask rather than through it. The experiments showed that if the number of threads in the warp of the gauze be added to the number in the woof and the sum multiplied by the number of thicknesses used, the resulting figure, to insure efficacy, should be at least 300. Experiments carried on by medical officers at the Rockefeller Institute for Medical Research indicated

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that gauze was a relatively inefficient material for the construction of masks.³⁸ They recommended the use of a three-layer mask of butter cloth. The Camp Grant experiments showed that if a mask be temporarily removed, allowed to dry, and then resumed in the reverse position it becomes a disseminator of bacteria rather than a filter. It was therefore recommended that each mask be marked in such a way as to indicate the side to be placed next the face. When worn for their protection by the uninfected the nose should be covered as carefully as the mouth.

Further experiments performed at the instance of the National Research Council tested various mask materials against a dry suspension of B. prodigiosus in air, the suspension being passed through a filter and the bacteria recovered in absorption bottles containing saline.³⁹The latter was then plated out and the count after incubation compared with that of the same suspension run without filtration as a parallel control. These experiments showed that the three-layer butter-cloth mask, shown by the experiments at the Rockefeller Hospital to be efficient in preventing the projection of infected droplets by the person wearing it, is by no means satisfactory in protecting the wearer from the inhalation of bacteria suspended in the air after the bacteria have lost their original coating of moisture. In different experiments from 44 to 76 per cent of the bacteria passed this mask when used dry. It was found that its efficiency was greatly increased by moistening, and they suggest the use of a mask based on this principle. This work suggests that the care used by many to avoid the use of a mask dampened by the breath or by perspiration was misapplied. These workers also tested certain grades of felt which they found to restrain the passage of bacteria perfectly. Masks of this material would have to be made over a frame to obtain sufficient filtering surface. There is no report of the actual use of masks of this design.

It is evident that the mask may be an efficient means of limiting the spread of infection. It is equally evident that unless the necessary conditions are fulfilled in the construction and wearing of the mask it may be useless and in some cases even harmful. For this reason statistics as to the results attained in the general use of the mask in the Army camps during the epidemic are of doubtful value. The classification of the camps in this respect follows:²²

		Percentage incidence of influenza			Percentage incidence of influenza
	Above average	Below average	Above average		Below average
Large camps:			Small camps:

19 using masks	9	10	16 using masks	11	5
15 not using masks	7	8	25 not using masks	13	12
Medium camps:			Total:

6 using masks	1	5	41 using masks	21	20
37 not using masks	20	17	77 not using masks	40	37

The mass statistical evidence shows no benefit in general masking. However, the considerations given above lead to caution in accepting this negative evidence. The expressions of opinions by medical officers are decidedly conflicting. It is apparent in the consideration of this matter, as in so many others, that the

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preponderant influence on camp rates of the relative proportions of recruits and longer service men, and, especially when comparing mortality, of geographical position, determines the camp rates and that other factors have had relatively little effect. No one has expressed the opinion that the mask properly used can do harm. Experimental evidence points to the probability of its usefulness. Practical experience shows that the necessary conditions for its proper use are rarely attained except when used by trained hospital attendants for their own protection. The conclusion is that the mask is probably of great value potentially but that the difficulties in securing its proper use by the mass of a command are such as to render its general employment of doubtful utility.

USE OF PROPHYLACTIC SPRAYS

The use of prophylactic sprays or gargles, not only by newcomers but as a general measure throughout the camp, was practiced in many commands. The antiseptics used varied greatly. Perhaps the most generally used were dichloramine-T, quinine solutions, and silver nucleinate or argyrol. The value of the measure and the relative value, if any, of the different solutions recommended have not been determined by controlled observations. When the spray was used in a camp, lightly affected by the epidemic, local opinion was favorable; when it failed to prevent a high incidence it was condemned as ineffective. There is no general evidence that prophylactic treatment reduced the incidence of disease in the commands employing it. The figures for the camps follow:²²

		Percentage incidence of influenza			Percentage incidence of influenza
	Above average	Below average	Above average		Below average
Large camps:			Small camps:

12 using prophylaxis	7	5	21 using prophylaxis	13	8
25 not using it	12	13	24 not using it	11	13
Medium camps:			Total:

13 using prophylaxis	7	6	46 using prophylaxis	27	19
17 not using it	8	9	66 not using it	31	35

These figures appear to indicate an increased incidence in camps using prophylactic methods. While, as indicated above, other factors may have been responsible for the difference shown in the two groups, the result is suggestive of possible danger in the use of general spraying. When used on large numbers of men the danger of conveying infection from throat to throat would appear to be very real, especially as the necessities of the case during an epidemic require that this work be delegated to hastily trained personnel. From Camp Funston, Kans.,³⁴it was reported that spraying of the nose and throat with antiseptics as a prophylactic for contacts and attendants predisposed to infection rather than protected. This was proved by a group experiment of 25 attendants on influenza cases who were not sprayed. One contracted the disease. Of 25 sprayed with protargol solution 17 contracted the disease. All 50 attendants wore masks and worked under the same conditions in the same temporary hospital.

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At Camp Cody, N. Mex., daily spraying was employed in many organizations but not in all.²²Its use was discontinued upon discovering the incidence of disease to be much greater in organizations in which its use was carefully employed than in others in which it was not used.

At Camp Upton, N. Y., a control experiment was carried out with two battalions of the depot brigade; 800 men were treated daily by spraying the nose and throat with a solution of dichloramine-T.³⁴A like number were held untreated as controls. Over a period of 20 days the incidence in the two groups was the same.

The experience of the epidemic thus shows that not only is there no evidence of benefit to be derived from the general use of the prophylactic spray but there is definite evidence from certain quarters that its use may at times distinctly increase the incidence of disease.

PROTECTION OF TROOPS FROM UNDUE FATIGUE

The effect of fatiguing drills and other duties, especially on newly joined recruits, has been the subject of considerable study. When a number of drafted men arrived at a training camp the necessary processes of enrollment and equipment, of physical examination, of vaccination against smallpox, and of inoculation against the typhoid fevers involved a period of several hours during which the men were constantly standing in line, much of the time with little or no clothing on. This strain after a long journey by troop train, often ending at camp during the night, and followed by the reaction from the inoculations has been held by some to be responsible for the high incidence of disease among recruits. Much of the fatigue and strain thus imposed upon the recruit is possibly a military necessity, although this is a debatable question.

During the summer of 1918 repeated waves of influenza and pneumonia occurred at Camp Funston, Kans., each wave in turn practically confined to recently inducted troops. In one such outbreak a study of the effect of variation in the training schedule was made.⁴⁰ Recruits were quartered in two separate parts of the camp, for convenience called A and B herein. Owing to local conditions the amount of drill and fatigue duty in camp A was not over half that in camp B. Owing to congestion at the receiving station the troops at camp A were held from two to four days before undergoing the ordeal of physical examination, inoculation, and equipment. During the first two weeks of camp life this contingent had 3.7 per cent of its strength reported sick, while the men at camp B on the fuller schedule reported 6.5 per cent during the same period. The author of the report suggested that "the whole period of inoculation be regarded as one in which the body is being called upon for a severe biological effort," that consequently all other effort should be reduced to a minimum, and that recruits should not be expected to reach a point where hard work on full time is possible for at least a month.⁴⁰

That the great difference in incidence among recruits as compared to seasoned troops is not the result of the inoculations against typhoid and paratyphoid is the conclusion drawn from a study of this point made at Camp Funston coincidentally with the work above mentioned.⁴¹It was shown that the curve of pneumonia incidence was a fairly regular one of the usual fre-

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quency type, with its highest point near the end of the second week in camp. There was no detectable relation between this curve and the dates of the typhoid vaccinations and no grouping of extra cases on or following the inoculation dates. This agrees with the result of a series of experiments on mice carried on at the Army Medical School which showed that animals inoculated with typhoid vaccine were less susceptible to streptococcus infection than were control animals.⁴²The pneumonia commission at Camp Wheeler, Ga., in the fall epidemic (1918) were unable to trace any relation between pneumonia incidence and inoculation dates.⁴³

The conclusion is that the association of disease with the inoculations is merely coincidental and that if the latter in any way increases susceptibility to respiratory infection it has proven impossible to demonstrate it by statistical methods.

That the Camp Funston figures, showing the effect of fatigue on disease incidence, are probably of general value is indicated by the numerous reports of high sick rates in newly inducted troops during the fall epidemic. Some of these have been noted above in the consideration of the effect of length of service. The effect of fatigue and exposure on seasoned troops is seen in the high proportion of pneumonias and the high case fatality of the American Expeditionary Forces.¹It seems well established, then, that during the prevalence of respiratory disease in a command, training schedules, especially those for the newer men, should be reduced to the minimum permitted by military necessity. Indeed it would seem that military objectives would in the long run be furthered by this course.

LIMITATION OF PUBLIC GATHERINGS

Almost all the large camps prohibited the gathering of large numbers of men indoors at entertainments and the like.⁴⁴Such a ruling would naturally follow the adoption of interorganization quarantine, which appears to be of distinct value in slowing the spread of an epidemic. An interesting instance of increased incidence of influenza following such gatherings was recorded at the San Quentin Penitentiary in California,⁴⁵where the weekly moving-picture show was shown to be followed regularly by an increase in the number of new cases of the disease. That gatherings indoors may also be the cause of dissemination of the virus is suggested by the fact that on the substitution of outdoor band concerts for the indoor show at this institution the number of cases two days later was still significantly larger than during the rest of the week. This suggests that the massing of men in close-order drill may have elements of danger. However, men associated in drill are usually together in mess and barracks, and the drill can hardly be expected to exert much extra influence.

USE OF THE CUBICLE

The consideration of the advantages of separating men's beds by means of hanging sheets or halves of shelter tents does not differ whether the system be used in hospital wards for the limitation of secondary infections or in the sleeping quarters of the men to prevent or limit the dissemination of bacteria during sleeping hours. Facts tending to show the value of the system in the hospital

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are equally applicable to its use in barracks. There is no evidence in the mass statistics that the lack of the use of the cubicles in the few hospitals that failed to utilize this precaution was of any influence on their mortality rates. Nor is there any statistical evidence that screening between beds in barracks lowered the rate of incidence. Indeed, of 19 large camps using the cubicle in barracks, 12 showed an incidence rate above the average to 7 below it; of 19 not using the screen, 7 were above the average and 12 below.²²However, if the average percentage incidence of the two groups is calculated it is found that the two figures are practically the same.

The studies of the special commission at Camp Pike, Ark., during the fall epidemic of 1918, furnished definite evidence as to the value of the cubicle in preventing cross infection in hospital wards.¹³Similar reports were made earlier from Camp Dodge, Iowa,⁴⁶and Camp Taylor, Ky.⁴⁷A report from Camp Hospital No. 1 at Camp Upton, N. Y., gives additional evidence.²²Due to lack of material certain beds in this hospital were not separated by sheets. The percentage of pneumonia cases among occupants of these beds was 23.36, while among those in cubicles the pneumonia incidence was 19.3 per cent. Such studies as these appear to indicate that the degree of isolation procured by the installation of the cubicle system is sufficient to have some effect on the distribution of bacteria.

Certain other measures having the same general object were adopted here and there. In some places sheets were hung down the centers of mess tables, or men were allowed to sit on one side only. Seats at mess were separated by a wider space than usual. The regulations as to the distance between beds and requiring the men to sleep head to foot were more strictly enforced. There is little evidence of any practical results from these measures during the severe epidemic.

In summary, of the measures instituted to prevent the entrance of influenzal infection into a camp and to limit its spread once it has obtained a foothold, it has been shown that strict quarantine may prevent the disease entirely. This is rarely practicable in large commands, and the most that can usually be expected is to delay the outbreak somewhat. Interorganization quarantine within the camp and the prohibition of unnecessary gatherings undoubtedly serve to diminish the explosiveness of an outbreak and to enable the individual cases to be better cared for. Medical inspection of commands daily or oftener, with prompt removal of discovered cases, should serve the same purpose. The use of the mask by the command in general, while theoretically sound, is beset with so many practical difficulties in application that until properly constructed masks can be supplied in quantity and their use in an efficient manner enforced, decisive results from their use can not be expected. Masking of hospital attendants and of patients has been shown to be of great value. The mask is more effective when moist than when dry. The use of prophylactic sprays has been shown to be not only useless but dangerous. Troops should be spared all unnecessary fatigue and exposure during an epidemic. The avoidance of crowding in barracks is undoubtedly of great importance, and the use of the cubicle in sleeping quarters as well as in hospital wards may be regarded as of proven value. When all is said, however, the best result to be expected from any or all

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of these measures is a slowing of the progress of an epidemic rather than any considerable diminution in the number of cases. The differences in admission rates of different commands depend primarily on differences in the relative numbers of susceptibles, mainly recruits. This being the case, the development of means of individual prophylaxis or immunization becomes of prime importance.

PROPHYLACTIC VACCINATION

With the coming of the severe fall wave of the influenza epidemic, attention was very generally directed to the possibility of individual protection by means of inoculation of bacterial vaccines. Though reports had indicated great uncertainty on the part of the bacteriologists as to the primary etiological relationship of the Pfeiffer bacillus to the disease, most vaccines used contained this organism. It was usually combined in varying proportions with type pneumococci, hemolytic streptococci, and even staphylococci. Many apparently favorable reports were made, but owing to the explosive character of the epidemic and its appearance nearly simultaneously in all parts of the country, most extensive vaccination experiments were made after the epidemic was on the wane or at least well under way. If, then, it be remembered that the case fatality is greatest during the earlier part of an outbreak, and if the results in persons vaccinated relatively late in an outbreak are compared with a control group whose cases and deaths were counted from the beginning, it is seen that it is easy to obtain figures more favorable to the vaccine than the facts warrant. Many such reports were published. The results of vaccination with any of the organisms used during the war period in reducing the incidence of the primary influenzal infection may be regarded as negative.

With the development of knowledge of the specific differences in the types of pneumococci, hopes were aroused that vaccination with the types responsible for the greater number of cases might reduce the incidence of pneumonia. The first large-scale experiment in this country was undertaken at Camp Upton, N. Y., in the spring of 1918.⁴⁸ Over 12,000 men were inoculated with a saline vaccine containing pneumococci, types I, II, and III. In the 10 weeks subsequent to this treatment the vaccinated men remained free from pneumonia due to these types, while the 19,000 unvaccinated men furnished 18 such cases. There was also shown a marked reduction in the rates of the vaccinated troops for Group IV pneumonias and especially for streptococcus pneumonias. The total pneumonia incidence was 1.33 per thousand for the vaccinated for the 10-week period, and 5.29 per thousand for the control group. It proved impossible to compare the groups further, owing to their departure for France. The vaccine used in this experiment contained equal parts of each of the three fixed types of pneumococci, 1,000,000,000 of each for the first dose, 2,000,000,000 for the second, while the third and fourth doses contained 3,000,000,000 each of Types I and II and 1,500,000,000 of Type III. The injections were made at weekly intervals, the majority of the men receiving 3 or 4 doses, some only 1 or 2.

A similar experiment was carried on at Camp Wheeler, Ga., in the fall of 1918.⁴⁹ In this instance the vaccine was a lipovaccine containing 10,000,000,000 cocci of each of the three fixed types, 30,000,000,000 in all. It was

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prepared at the Army Medical School. It was given in one dose of 1 c. c. The reactions, general and local, were not unduly severe and no serious disability resulted therefrom. The troops vaccinated included both white and colored, both seasoned men and recruits. The period of observation following the inoculation included the period of the fall epidemic of influenza. The results are tabulated below. The vaccination of the older men had been accomplished prior to the arrival of the recruits. The latter were inoculated immediately on arrival in camp.

Recruits (less than one month's service):		Seasoned men:
White-	Per thousand	White-	Per thousand
Vaccinated	38.6	Vaccinated	5.2
Unvaccinated	64.0	Unvaccinated	40.0
Colored-		Colored-
Vaccinated	125.0	Vaccinated	14.8
Unvaccinated	413.0	Unvaccinated	78.4
Total recruits-		Total seasoned men-
Vaccinated	58.2	Vaccinated	7.2
Unvaccinated	115.2	Unvaccinated	46.4

The same pneumococcus lipovaccine prepared at the Army Medical School was tried to a considerable extent on volunteers in other camps following the promulgation by the Surgeon General of a circular letter, dated October 25, 1918, authorizing its general use. It was not used, however, until a time when the accurate estimation of results was interfered with by the shifting of troops incidental to the demobilization. Favorable reports were received from the vaccination of large numbers of men at Camps Funston, Kans.; Dodge, Iowa; Dix, N. J.; Sherman, Ohio; Wadsworth, S. C.; and Devens, Mass.²²In Camp Devens, it was estimated that the pneumonia rate in vaccinated men was about one-fourth that of the unvaccinated. Camp Custer, Mich., reported unfavorable results on a small group. On the whole the reports received from camps in this country were decidedly favorable. In the American Expeditionary Forces, a carefully controlled experiment was made at Camp Lusitania.⁵⁰ Here, 5,000 men were vaccinated and 3,861 held as controls. Several varieties of vaccine were used, a lipovaccine containing pneumococci types I and II, and one containing all three types, both having been prepared at the Army Medical School, and a saline vaccine prepared by the Pastuer Institute, Paris, containing "pneumococci, streptococci, staphylococci and B. influenz?." There was little respiratory disease in the command during the period of observation January to June, 1919. The results, however, indicated that the lipovaccines reduced the incidence of pneumonia to about one-fourth that of the controls. The saline vaccine showed no such result. All vaccines showed a reduction in the incidence of influenza and common respiratory diseases, the saline vaccine in this instance showing as good a result as the others. A careful series of serological observations was made on representative numbers of the vaccinated men. Blood was taken from these men semimonthly for the period of observation. Antibodies were demonstrated for pneumococci types I, II, and III, beginning the second week after inoculation, reaching a high point at the end of four weeks, then gradually decreasing during the balance of the period. The response to type I was most marked, type II next, and

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type III least. The sera were tested by agglutination and by complement fixation methods. The men inoculated with the saline mixed vaccine gave substantially the same reactions as those on whom the Army Medical School lipovaccine was used. Protection experiments with mice also demonstrated the value of the treatment.

SUSCEPTIBILITY AND IMMUNITY

During a great pandemic outbreak of influenza the disease is so widespread and affects so large a proportion of the population at one time as to lead very naturally to the impression that practically 100 per cent of the population has been exposed to the disease. If this be so, we must assume that certain individuals, perhaps the majority, possess immunity against this infection, as the figures do not indicate that the entire population becomes infected. The figures given in previous pages, which, as stated, probably constitute a minimum estimated number of the cases of respiratory disease that can be attributed to the influenza virus in the Army during the World War, show that 26.6 per cent of the men in the military service contracted some form of this disease. The reverse of this proportion is that 73.4 per cent, while equally exposed, escaped infection. It would appear that practically three-fourths of the men, living under conditions as favorable to the transference of infection as can well be imagined, failed to contract the disease. Does this mean that these men were naturally refractory to this infection, that they acquired an immunity at some prior date, or that they failed to come in contact with the active virus? That the lost supposition could be true to any considerable extent in the Army seems too improbable for argument; it might have held some place in the population at large, where the individuals are not in such constant contact with each other and particularly are not habitually associated in relatively large groups. It may be safely assumed for present purposes that practically every soldier had the opportunity to contract influenza if his physical condition was such as to render him susceptible to it. Assuming this, then we are forced to choose for our explanation of the immunity shown by the majority of the men one of the other two possibilities though granting that both may have had their part in producing the effect. The possibility must be borne in mind that many cases may have occurred of such mildness as to have attracted no particular attention, but still leaving an acquired immunity. This is largely an academic distinction, however, as such persons must have possessed marked resistance to the infection, or its manifestations would have been more severe. The disease common among us which in many ways is most like influenza is measles. Immunity to this disease is rarely congenital. It is generally admitted that practically all persons are susceptible to it unless protected by previous attack. With measles, however, the immunity conferred by an attack is usually permanent. Bearing in mind the analogy of measles, we should naturally feel that immunity to influenza was most probably acquired and due to a previous attack, but here we find that the question of acquired immunity itself has been called into question.

Earlier writers on the subject were inclined to deny the existence of immunity after an attack of influenza. Parkes⁵¹ in 1870 said, "There is some

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discrepancy of evidence; but, on the whole, it seems clear that, while persons seldom have a second attack in the same epidemic (though even this may occur), an attack in one does not protect against a subsequent epidemic." This opinion has been repeated in substantial agreement by many authorities since, and it is a familiar observation that the same individual may have repeated attacks of "grip" from year to year. Whether these repeated attacks are indeed due to the same virus is, as has been said before, still open to argument. On the other hand, it seems necessary to assume that convalescence from influenza involves some degree of immunity, as otherwise we should be faced with a condition wherein each susceptible person would contract attack after attack in rapid succession. Moreover, the usually accepted explanation for the passing of an epidemic wave, that of the exhaustion of susceptible material, depends on the assumption that those who recover from the disease are at least temporarily immune. While we must grant that such immunity in the case of influenza does not last for life, or even perhaps for any considerable period of time, a number of observations were recorded during the World War that tend to throw light on this problem and to make possible a fairly definite answer. First, we have the general observation that troops who passed through the epidemics in the winter and spring preceding the great outbreak of the fall of 1918 showed a decidedly lower attack rate than was the case in the newer troops. To give value to this observation we must assume that these earlier outbreaks affected the Army more extensively than they did the general population. This is a fair assumption, as we have shown that this was true to a considerable extent even in the fall of 1918. We should also have to discount the effect of "seasoning," in so far as this may be shown to be a nonspecific immunizing process, as suggested by the analogy between the effect of service and of relative urbanity of troops, as shown above. Hence the relative immunity of the American Expeditionary Forces, for instance, may have been due more to a nonspecific seasoning process than to the development in its men of a specific immunity. If this be true, it is fair to add that the degree of protection thus afforded is probably as great as could be expected from this process. The average admission rate for the American Expeditionary Forces from respiratory diseases was 143.4 per 1,000, the corresponding rates for the troops in this country was 227.7, a figure about one half again as high as that of the American Expeditionary Forces.¹If we can show for various units a degree of protection greater than this following a previous outbreak of the disease we shall be justified in assuming that specific immunity entered into the case.

At Camp Shelby, Miss., there was in April, 1918, a division of troops numbering about 26,000.⁵² An epidemic of mild influenza struck this camp at this time, and within 10 days there were about 2,000 cases, including not only men who were sent to the hospitals but also men who were cared for in barracks. This was the only division that remained in this country from April until the fall of 1918.

During the summer this camp received 11,645 recruits.⁵²In late August, 1918, the virulent form of influenza struck this camp. It confined itself almost exclusively to the recruits of the summer and scarcely touched the men who had

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lived through the epidemic of April. Not only the 2,000 who had the disease in April but the 24,000 who apparently were not affected escaped the fall epidemic. Vaughan stated:⁵³ "It appears from this that the mild form of influenza of April gave a marked degree of immunity against the virulent form of October."

This observation points to the existence of both possible types of immunity: A natural type possessed by the body of the above command that failed to contract the disease on either exposure, and an acquired type in those who passed through the April attack.

The surgeon of the 11th Regiment of Engineers, A. E. F., reports in some detail a parallel occurrence.⁵⁴During May and June, 1918, this organization, already a seasoned body of men, was attacked by an epidemic of influenza which involved 613 men in a strength of about 1,200. There were two deaths from pneumonia. Company B, the unit first attacked, had almost all of the cases for the first two weeks, when the other companies were also attacked. This regiment thus showed an attack rate of over 50 per cent at this time, the company first attacked showing the lowest incidence. During the succeeding five months, the period of greatest mortality from influenza, this regiment was working in the St. Mihiel and Argonne sectors. About 150 men had colds of varying degree, usually attributed to the conditions under which they were living and working. There were 3 cases of pneumonia, of which one died. The regiment thus passed through the worst of the influenza epidemic with practically no sickness. In early January, 1919, the regiment was grouped at Commercy and moved to Bordeaux for shipment home. Here it was again attacked by influenza, then present in the civilian population with a daily mortality of about 1 to 2,000 inhabitants. During January and February there were 270 cases of influenza in the regiment, with 35 cases of pneumonia and 5 deaths.

These cases tabulated by companies with those of the earlier outbreak show that the companies that suffered most in May and June, 1918, had the least disease in January and February, 1919.

		May and June, 1918, influenza	January and February, 1919
	Influenza	May and June, 1918, influenza	Pneumonia	Deaths
Company A	78	65	9	2
Company B	74	120	18	3
Company C	126	25	4	0
Company D	128	---	---	---
Company E	123	60	4	0
Company F	84	---	---	---
Total	613	270	35	5
Percentage of command	51	22	---	---

The regimental surgeon stated: "One feels justified in assuming that the early epidemic had conferred sufficient immunity to keep the regiment free from influenza for six months, and partial immunity extended through January and February." He noted further, that some of the men attacked in the winter had also had influenza in May or June, but as a rule these proved to be mild cases.

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Vaughan⁵³ says: "Those who had influenza in September and October were not affected by the recurrent waves. In the recurrence of influenza at Dorr Field during January, 1919, Squadron A, which was affected most severely in the first epidemic, had no cases. From Camp McClellan we get a report of a similar incident. Speaking of influenza during December, the report goes on to say that Battery A, which had very few cases during the October epidemic, sent more cases to the hospital than any of the other units in this organization, while Battery G, which was affected most during the epidemic, had very few cases this month. Camp Jackson reported a recurrence of influenza in January, which was localized largely in the 48th Infantry, a regiment which had passed through the autumn epidemic with very few cases at Camp Sevier."

An interesting comparison is reported from Camp Dodge, Iowa.⁵⁵In this case the influence of seasoning, or length of service, may be eliminated. There were in this camp at the time of the fall outbreak, two regiments of Regular Infantry. The 2d Infantry had been in Hawaii and there had encountered the earlier wave of influenza, reporting 300 cases. The 14th Infantry, partly from Alaska and partly from Fort Lawton, Wash., arrived in camp during the fall outbreak, with no history of previous exposure to the disease. The 2d Infantry reported influenza in 6.6 per cent of its strength, the 14th Infantry, in 48.5 per cent. They were indeed the organizations having the lowest and highest incidence of all camp organizations, respectively. If this observation be of general significance it would point to the fact that seasoning in regard to this disease is a specific rather than a nonspecific process.

These more or less fragmentary observations might be multiplied almost indefinitely, but enough has been quoted to show that an organization which had passed through one outbreak of influenza was much less likely to suffer as severely a second time, indeed had apparently, for some months at least, received a substantial measure of protection, much more than difference in length of service would imply, especially as in cases like that of the Engineer regiment described above the organizations were already seasoned when the first epidemic appeared. It would appear, however, that in any organization there are many men who do not take the disease in recognizable form, even in a succession of epidemics.

The question of the duration of the immunity acquired by an attack of influenza becomes of great interest at this point. We have seen above that it is the consensus of opinion among authorities on the subject that an attack in one pandemic outbreak is powerless to protect against another attack years thereafter. Some, indeed, as West, think that an attack predisposes to subsequent attacks:⁵⁶"It seems more likely that an individual may never have influenza at all than that having had it once he should never have it again." It appears entirely probable from what has gone before that a definite specific immunity is induced by an attack of influenza. That this immunity is not of long duration appears equally well established. The instance of the 11th Regiment of Engineers serves to illustrate both these points.

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THE INFLUENZAL CYCLE DURING THE WAR PERIOD

It was pointed out above that we are apparently justified in assuming the incidence, during the period studied, of seven separate waves of acute respiratory disease associated with pneumonia. The first of these, coming before the mobilization of the National Army, is perhaps the most doubtful and careful studies of its characters are lacking. With the advent in December, 1917, of the second well-defined wave of this character, attention was forcibly drawn to the situation by the occurrence of large numbers of cases of pneumonia with a high mortality. These were at first regarded as secondary to the epidemic of measles which was then on its decline, but as the number of cases of pneumonia continued to increase it became evident that many, if not most of them, had no relation to the former disease. The fact that there were, concurrently, large numbers of cases diagnosed as influenza, bronchitis, etc., was not at the time given the significance that study of the relations between this class of diseases and the pneumonias shows to be its due. Indeed, a comparison between the incidence of acute respiratory disease and the case fatality of measles or with the percentage of measles cases developing pneumonia, shows such a marked degree of correlation that it seems more than probable that even the post-measles pneumonias were due in large measure to concurrent infection with the virus of influenza. Chart XXIII shows these relations graphically. Such a relation would serve to explain the unprecedented high incidence and fatality of measles pneumonia.

It has been shown that groups exhibiting greater susceptibility to infection by the influenza virus also tend to show a greater proportion of lobar pneumonia, while the groups more resistant to the primary infection show increasing proportions of bronchopneumonia. In the gradual evolution of virulence and invasiveness on the part of the virus by which it worked up to the peak of its activity in September and October, 1918, it is probably true that in each successive wave of increasing invasiveness it attacked the most susceptible of the soldiers first, and that each wave attacked men whose resistance could be overcome at that stage of its evolution, leaving those that recovered temporarily immune from later and often more fatal attacks of the disease. With each wave the relation between the virulence and invasiveness of the virus, on the one hand, and the susceptibility or resistance of the population, on the other, determined the incidence and fatality of the disease.

The pneumonias of the first winter (1917-18) of the period under consideration resembled those of the interepidemic period more closely than was the case later, and the proportion of lobar pneumonias was greater than in subsequent outbreaks. Bacteriologically, the findings in pneumonia sputa and lungs varied greatly in different camps. Thus in Camp Travis, Tex., the majority of the winter pneumonias were associated with infection by the hemolytic streptococcus, while at Camp Wheeler, Ga., which also had a very high pneumonia rate at this time, this organism did not make itself felt until the advent of the third or spring wave of infection, the pneumococcus in its various types being found in the pneumonias and empyemata.²

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It is shown below, in the consideration of the etiology of the pneumonias, that, during this early period, the pneumococci which were found, though exhibiting a considerably larger proportion of the so-called mouth types, Types IIa,

CHART XXIII.-A comparison of the variations in the annual admission rates for the total respiratory diseases, the case fatality of measles, and the percentage of measles cases developing pneumonia, white enlisted men in 36 large camps in the United States, October, 1917, to March, 1919

III and IV, than was the rule in preepidemic times, still showed a much smaller proportion of these types than was the case later when influenza had attained its maximum virulence. The proportion of the fixed epidemic types, I and II,

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was correspondingly larger at this time. These latter types are undoubtedly more invasive than the mouth types and so require less predisposition on the part of their victims, while later, when resistance was still more reduced by infection with the more virulent virus, the invasion by mouth types became almost universal.

At this time the high incidence of respiratory disease accompanied by pneumonia obtained among the troops in France as well as in the United States. Though the curve began to rise in both groups at the same time, the peak was reached in the American Expeditionary Forces a month earlier than was the case at home. Our small body of troops in France at this time did not cover as much territory as was the case later and the infection could involve the whole command more quickly than was possible in the widely separated camps at home. Probably for the same reason the peak was higher, though the duration of the outbreak was shorter than in the United States. In both groups at this time the death rates were very much higher for colored soldiers than for white, the disparity being greater in France than at home. The admission rates for colored troops were also much higher abroad but at this time were about the same as those for white troops in this country.

Both here and abroad there was a decided drop in the rates during the month of February, 1918. Taking any one small group, such as a single camp, the interval between waves is seen to be greater than the one month, but owing to the fact that the outbreaks varied in their time of onset and subsidence in the different camps, sometimes by several weeks, the curve for the whole is smoothed and the interval between waves, shortened. In March, 1918, there began to be observed in the United States decided epidemic outbreaks of respiratory disease that was generally called influenza. Descriptions of the disease at this time both here and abroad leave no doubt as to the clinical and pathological identity of the epidemic with that which appeared in the fall. The immunity later shown by groups which passed through this spring outbreak shows that the infection was the same. It was, in most camps, explosive in its onset and it involved a large proportion of the men in each camp attacked, though in the majority, by no means as many as was the case in the fall wave.

Of 36 large camps in the United States, 24 showed a distinct peak of acute respiratory disease occurring either in March or April.¹ The other camps showed increases but in such a way as to divide their cases between the two months. The rates for the concurrent pneumonia present a strict parallelism to those of the acute respiratory disease as is shown in Chart XXIV. Now, too, for the first time, pneumonia was recognized as secondary to influenza in considerable numbers of cases. In several camps, Camp Wheeler, Ga., and Camp Dodge, Iowa,⁴⁴for instance, the character of the secondarily invading organisms showed a decided change, the hemolytic streptococci replacing the pneumococci with increasing case fatality. At this time the fulminating pneumonia, with wet hemorrhagic lungs, fatal in from 24 to 48 hours, was first observed. This was regarded at the time as characterizing the streptococcus at the height of its virulence. Later experience showed the lesion to be influenzal. The 24 camps having a distinct peak month (admission and death rates) are shown in the following tabulation; the month of highest incidence is also indicated for each camp:

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Admission and death rates in 24 camps exhibiting a distinct peak month during the spring epidemic of influenza, 1918

		Camp	Month	Admission rate	Death rate		Camp	Month	Admission rate	Death rate
1	Funston, Kans.	March	1,907	26.99	13	Greene, N. C.	April	775	10.20
2	Wheeler, Ga.	April	1,486	12.46	14	Upton, N.Y.	March	652	10.37
3	Shelby, Miss.	...do...	1,420	1.50	15	Beauregard, La.	April	649	3.72
4	Logan, Tex.	...do...	1,184	2.30	16	Gordon, Ga.	...do...	612	4.66
5	Dix, N. J.	March	1,180	7.06	17	Jackson, S. C.	...do...	584	.84
6	Dodge, Iowa	...do...	1,172	25.80	18	Meade, Md.	March	581	1.60
7	Fremont, Calif.	April	1,045	.88	19	Lee, Va.	April	565	2.70
8	Sheridan, Ala.	...do...	908	3.70	20	Kearny, Calif.	...do...	560	2.40
9	Bowie, Tex.	...do...	896	3.56	21	Sevier, S. C.	...do...	425	3.90
10	McClellan, Ala.	...do...	875	1.70	22	Wadsworth, S. C.	...do...	381	4.83
11	Sherman, Ohio	...do...	837	8.30	23	Greenleaf, Ga.	...do...	371	.00
12	Doniphan, Okla.	March	785	9.22	24	Hancock, Ga.	...do...	284	.93

In this tabulation the camps are arranged in the order of admission rates, beginning with the highest. It is seen that the death rates and admission rates are not closely correlated. If the rates of the camps showing a March peak are compared with those in which the peak came in April, the following figures are obtained.

Month	Number of camps	Death rate	Admission rate	Fatality
March	6	12.9	993	1.3
April	18	3.86	759	.52

Thus the camps having an April peak had nearly as high an admission rate, but much lower death rates and case fatality, than camps having a March peak. This parallels the relation found in September and October, as will be seen later. The order in which these camps were attacked roughly corresponds to the sequence of attack in the fall outbreak. Almost all the camps in the lower half of the table are southern camps. Of the southern camps having high death rates, Camp Wheeler, Ga., Camp Doniphan, Okla., and Camp Greene, N. C., all had high rates during the winter and were evidently composed of highly susceptible material. In this wave, for the first time, men from Northern States were seriously affected.

This wave of the epidemic was very much less explosive in the American Expeditionary Forces.¹ The rise after the February remission was less noticeable and the incidence of influenzal infections with some complicating pneumonias continued well into the summer. The mild character of the disease, together with lack of agreement as to its exact nature led to the designation of "three-day fever," by which it was generally known at that time. The fact that the troops were now widely scattered for training purposes doubtless made its spread less rapid.

It is seen, then, that undoubted influenza appeared at this time in both Europe and America so nearly at the same time as to render its transference from one area to the other very unlikely. That it was present as well in other parts of the world is indicated by a report of the camp surgeon at Camp Kearny, Calif., who attributed the outbreak at that station to the visit of a Japanese fleet which arrived with several cases on board.⁵⁷During the spring epidemic,

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both here and abroad, colored troops suffered decidedly more than did white troops. Following the outbreak of March and April there was a marked fall in admission and death rates for respiratory diseases in the United States. As has been stated above, the disease remained sporadically active in France

CHART XXIV.-The relation between the total respiratory diseases and the pneumonias, annual admission rates per 1,000 strength for the 24 large camps in the United States which showed a definite peak for these diseases in the 1918 spring epidemic. The figures have been combined so that the peak months in all the camps coincide

throughout the summer. Moreover, in this country there are indications that the disease was not absent but was smouldering, here and there attacking the susceptible individual. This is shown in the occurrence of many cases of pneumonia, even in June, the month of lowest incidence. Whenever a group of susceptible individuals was found there was apt to be an influenzal outbreak

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with a varying proportion of pneumonia. The summer of 1918 was one of troop movements on an enormous scale. Camp after camp sent its trained division abroad, and was filled up with recruits. Before leaving, many divisions received recruit detachments to complete their strength. At Camp Funston, Kans., several such drafts were received during the summer, each one within a few days of its arrival in camp being attacked by influenza and pneumonia.⁴¹ These recurring epidemics in recruits were carefully studied by a special board and a full report was published.⁴¹At Camp Cody, N. Mex., in June, the incoming draft suffered from "bronchitis" and pneumonia.¹⁶The same sequence was observed elsewhere to a greater or lesser extent. It has been shown above that the rates for influenza and pneumonia began to rise early in August, and increased progressively from that time to the high point in the fall. It is noticeable that the mortality rate of these cases increased through the warm months of the year, when mortality from respiratory disease is at its minimum.

In France, following the spring outbreak, there was a gradual decline in the rates for the American Expeditionary Forces as a whole, individual organizations having sharp epidemics.¹ July showed the lowest rate for the summer. Observers who investigated these different outbreaks expressed the opinion that there was a progressive increase in the severity of the disease as time went on. This is well illustrated by the series of epidemics occurring in the Artillery training camp at Valdahon.⁵⁸To this camp brigades of Field Artillery were sent for special work, one leaving as another arrived. Early in July, 1918, an outbreak was reported here and was investigated by officers from the office of the director of laboratories and infectious diseases, Dijon. This mild epidemic ran its course in a few weeks and the disease subsided until a new brigade entered the post. Within a few days influenza broke out among these troops, affecting especially organizations that occupied the barracks vacated by the companies that had shown the largest number of cases in the first outbreak. As there was little chance for contact between the troops and the permanent camp personnel, it is probable that the virus persisted in the barracks. Later the disease broke out in a third brigade in spite of careful disinfection of the barracks, in this instance being somewhat delayed in its onset. With each outbreak in visiting troops there was an increased number of cases among the permanent personnel. Troops in neighboring towns, even one regiment that marched into the camp daily for firing practice, failed to contract the disease.

That this progressive increase of invasiveness and virulence on the part of the influenza virus was also taking place in the United States is shown by the progressive increase in case fatality during the summer months shown in the general tables and charts. An occurrence somewhat similar to the Valdahon epidemic took place at Camp Shelby, Miss., beginning August 29, 1918.⁵⁹ The Shelby outbreak was a sharp explosive one involving, as stated elsewhere, only those men who had not passed through the spring epidemic. It was not as fatal as later waves in the same camp, probably due to the importation of cases from the North, but attack at this time afforded protection against infection with the more virulent strain. There was a decidedly higher fatality than had been observed in the summer outbreaks. This outbreak and the subsequent waves occurring with the advent of new men and practically limited to them are shown in Chart XXI.

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This progressive increase in virulence and invasiveness culminated in the explosive outbreak of the autumn wave of disease which reached epidemic proportions early in September and reached its peak in the following month. Again the increase in cases seems to have affected both the American Expeditionary Forces and the Army at home at very nearly the same time. The differences in incidence between the two main portions of the Army, that is, at home and in France, are shown in the general tables and charts. In this country the negro troops showed a relatively lower incidence at this time than before; indeed their admissions throughout this outbreak and their death rate during September were lower than that of the whites. The case fatality, however, in colored troops is consistently higher than that for white troops. Both races showed higher admission and death rates for the country as a whole in October than they did in September. The case fatality in whites was higher in September, while the October fatality was higher in the colored.

It is probable that it will never be definitely settled where the severe and fatal form of influenza arose in the fall of 1918. Each station, with few exceptions, claimed to be able to show that it owed its disease to transmission of infection from some previously affected point. Camp Devens, Mass., the first to report the highly fatal type of the disease,⁶⁰ is supposed to have been infected from the city of Boston. The first cases there were on the naval receiving ship at Commonwealth Pier, August 28; thence the disease spread and soon infected the city. It is customary to think of all the subsequent influenza as having spread from this point and in many cases it is possible to trace this spread with some definiteness. There are some facts, however, that tend to the belief that there was a general increase of the activity of an already widely distributed virus which might have given rise to the fatal wave independently of infection from abroad.

Of these facts the first is that already mentioned, that from early August the influenza rates for the Army showed a progressive increase in geometrical ratio, the curve of rise plotting out on logarithmic paper as a practically straight line. (Chart XIV.) The second is the occurrence of definite outbreaks of increasing severity during the summer in this country and especially in the American Expeditionary Forces. These outbreaks ran right up to, if they did not indistinguishably blend with the great fall wave. The case of Camp Shelby, Miss., has been discussed. Camp Logan, Tex., is another camp whose outbreak appears to have been entirely independent of the Boston strain of virus, at least in its early stages.⁶¹The disease appeared here just two days after its advent at Camp Devens, Mass. There is no known means by which the infection could have been transmitted from Camp Devens, and the disease did not make its appearance in the civil population of Texas for some time thereafter. The possibility of this outbreak having been due to a strain of the virus less virulent than that active in the Northeastern States is indicated by the fact that the fatality at Camp Logan was less than half that at Camp Devens and the further fact that the outbreak is recorded as having been of little severity until the receipt of recruits from the North already infected, who appeared to bring with them a more virulent infection. Camp Cody, N. Mex., had a similar experience in this respect.⁶²

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Another circumstance tending to support the view that the disease may have evolved its virulent type more or less generally throughout the world rather than that the virulent strain arose in one place and spread by contact, is the apparent impossibility of tracing this spread with any definiteness. As stated above, most stations regarded the disease as imported from without. In the American Expeditionary Forces it was felt that the severe influenza was brought from America. The first of the severe cases at Brest, France, for instance, occurred in a replacement detachment from Camp Pike, Ark., shortly after landing and while occupying a relatively isolated camp of their own.⁶³This detachment had left America late in July, 1918, and at a time when the incidence of respiratory disease was near its lowest here. The outbreak was of a severe type, with a high mortality, and for some time was limited to this detachment. Numerous reports of outbreaks in the American Expeditionary Forces during the severe wave express the belief that the disease was introduced by replacement troops from the United States.⁶⁴On the other hand, the general belief here was that the fatal influenza was introduced from Europe through the port of Boston. It is possible, of course, that neither view is correct, but that the fatal strain originated somewhere else and was introduced to both theaters nearly simultaneously. This is suggested by the report that a Norwegian vessel landed several severe cases in New York early in August, having suffered greatly from the disease on her voyage; however, there was no suggestion of fatal influenza in Norway at this time.⁶⁵ The fatal type must have evolved somewhere from the less virulent variety. Its essential indentity with the latter is shown by the facts of immunity already recorded. It appears possible that this evolution might have occurred in many places at nearly the same time. An observation along this line is the fact that in several camps in the United States the outbreak of fatal influenza was preceded for several days by increasing incidence of mild cases gradually changing to the severe type. The introduction of a fully virulent virus from without would naturally be expected to produce fatal infections from the start. This transition is described for Camp Sherman, Ohio, as follows:⁶⁶

At the time the prevailing epidemic was at its height in New England numerous cases of coryza and bronchitis appeared at Camp Sherman. The picture was not characterstic of influenza, but the condition was so frequently noticed among patients at the base hospital that isolation was instituted and special wards set aside for this purpose. The absence of the usual features of influenza led to considerable comment as to the justification of such a diagnosis. This uncertainty was abruptly and definitely terminated by the sudden appearance of large numbers of patients exhibiting characteristics of clinical influenza. * * * By September 24 the transition had occurred.

Somewhat similar observations are recorded for several other camps.

These facts, then-the undoubted general distribution of the disease for months preceding the great outbreak and its progressive increase in prevalence and fatality from early in August, the difficulty of showing with any definiteness where the fatal type originated, and the suggestion in some reports of the development of a severe type locally from a milder strain-render it at least possible, and even probable, that the severe form arose not as a single strain but that many strains acquired increased virulence in different places at about the same time. The question remains unsettled, and more detailed studies of

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future outbreaks will be required for its answer. Whichever view ultimately prevails, there can be no doubt that in many stations the disease was definitely introduced from without. Once introduced, the disease spread with characteristic rapidity and involved a large proportion of the command.

Table 27 shows the main facts of the epidemic for the large camps in the United States. It is impossible to give details of these outbreaks. The differences in incidence and fatality, as influenced by various factors, have already been discussed. It is desired, however, to describe the outbreak in one or two camps in order to give an idea of the conditions prevailing and of the difficulties faced, and to a great extent overcome, in the care and treatment of such enormous numbers of very sick men at one time.

TABLE 27.-Influenza and pneumonia. Admissions, deaths, and case fatality rates, for 40 large camps in the United States during the fall epidemic, 1918

Camp or station^a	Epidemic period	Admission			Mortality-Influenza and pneumonia		Case fatality rates, per cent
Camp or station^a	Epidemic period	Influenza and pneumonia-absolute numbers	Influenza, per cent of strength	Pneumonia, per cent of strength	Absolute numbers	Per cent of strength	Influenza	Pneumonia
Sherman	Sept. 24-Nov. 19	13,161	33	6.6	1,101	3.3	10.0	51
Cody	Sept. 26-Dec. 12	4,040	44	11.6	240	3.3	7.5	29
Beauregard	Sept. 18-Oct. 20	8,551	53	10.1	410	3.0	5.7	30
Grant	Sept. 21-Nov. 3	13,071	26	5.7	1,060	2.6	9.9	45
Dodge	Sept. 18-Oct. 22	11,931	30	5.8	702	2.1	7.0	36
Forrest	Sept. 29-Nov. 11	3,170	30	5.6	181	2.1	7.2	38
Dix	Sept. 9-Nov. 1	13,733	26	4.0	808	1.8	6.9	41
Devens	Sept. 8-Oct. 29	17,400	33	6.3	787	1.8	5.4	28
Meade	Sept. 17-Oct. 20	14,280	27	6.8	763	1.8	6.7	27
Custer	Sept. 23-Nov. 3	12,773	26	5.9	660	1.7	6.3	28
Jefferson Barracks	Sept. 28-Nov. 2	2,392	29	5.0	119	1.7	5.8	34
Greene	Sept. 22-Nov. 4	5,221	28	3.7	269	1.6	5.9	43
Funston	Sept. 16-Nov. 7	16,983	28	4.6	841	1.6	5.8	35
Humphreys	Sept. 13-Oct. 18	5,408	16	4.4	413	1.6	10.0	35
Syracuse	Sept. 12-Oct. 15	2,761	18	3.6	208	1.6	9.1	44
Upton	Sept. 13-Nov. 30	7,921	22	4.6	432	1.5	6.6	31
Lee	Sept. 13-Nov. 10	13,597	24	3.9	674	1.4	5.8	34
Greenleaf	Sept. 23-Nov. 26	6,159	22	4.2	325	1.4	6.3	33
Bowie	Sept. 26-Nov. 13	5,212	43	6.8	142	1.4	3.2	20
Hancock	Sept. 28-Nov. 4	8,984	22	3.6	499	1.4	6.5	39
Sevier	Sept. 20-Nov. 3	5,422	16	3.1	340	1.2	7.5	37
Taylor	Sept. 22-Nov. 3	14,761	20	4.1	720	1.2	5.9	29
Wheeler	Sept. 30-Nov. 29	2,876	18	4.8	140	1.1	6.1	23
Jackson	Sept. 18-Oct. 16	9,427	22	3.1	412	1.1	4.9	35
Fremont	Oct. 8-Nov. 7	2,778	16	2.5	149	1.0	6.2	40
Eustis	Sept. 25-Nov. 19	2,352	18	2.6	118	1.0	5.8	40
Johnston	Sept. 18-Nov. 1	3,360	15	4.5	165	.95	6.4	21
MacArthur	Sept. 30-Nov. 4	8,354	35	3.3	189	.87	2.5	26
Logan	Sept. 10-Oct. 30	4,947	34	4.0	111	.86	2.5	21
Pike	Sept. 23-Oct. 31	13,124	23	2.5	423	.83	3.6	31
McClellan	Sept. 20-Nov. 8	5,445	14	3.7	228	.74	5.3	20
Kearny	Sept. 24-Dec. 8	5,188	25	3.6	129	.71	2.9	19
Sheridan	Sept. 28-Nov. 4	5,155	20	2.1	145	.62	3.1	29
Travis	Sept. 19-Nov. 9	12,120	28	7.2	199	.58	2.1	8
Wadsworth	Sept. 24-Nov. 4	1,883	10	3.0	80	.56	5.6	19
Las Casas	Oct. 20-Dec. 15	2,403	16	2.8	69	.52	3.4	19
Gordon	Sept. 19-Oct. 31	6,689	17	1.8	166	.46	2.8	25
Lewis	Oct. 9-Oct. 31	3,851	8	2.5	127	.35	4.3	14
Ellington Field	Sept. 30-Oct. 23	1,205	23	1.1	15	.30	1.3	26
Shelby	Aug. 26-Nov. 10	2,803	16	1.5	48	.29	1.9	19

^aArranged in order of mortality rate, influenza and pneumonia.

The outbreak at Camp Upton, N. Y., was studied and recorded in a way particularly valuable for epidemiological review.⁶⁷ The report states that the disease was brought from Camp Devens, Mass., by troops from that station and began abruptly with the admission of 38 cases on the first day, September 13, 1918. It reached its peak October 4, with the admission of 483 cases, and

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then rapidly declined. The following table shows the admissions by days for a period of seven weeks, the percentage of those admitted each day developing pneumonia, and the percentage dying. This classification is particularly valuable in that the cases admitted on a certain day are thus followed to their final disposition, and no allowance for the lag between admission and death is necessary, as is the case when deaths are recorded as of the day of occurrence. This table shows the abruptness with which the disease struck and gives an idea of the problem thrust upon medical authorities in the care of such numbers. It is noted that there is considerable irregularity in the figures for those developing pneumonia and dying. This daily variation is undoubtedly due to the fact that the disease attacked the organizations of a camp seriatim and that certain organizations contained more susceptible material than others. On the days when the admissions were mainly from susceptible organizations the resulting fatality was high, and vice versa.

Grouping the figures by weeks, an interesting relation is developed, thus:

Epidemic of influenza at Camp Upton, N. Y. Admissions by days, and percentage of daily admissions developing pneumonia, and percentage dying

Date	Admissions	Percentage developing pneumonia	Percentage dying	Date	Admissions	Percentage developing pneumonia	Percentage dying
Sept. 13	38	26	10.5	Oct. 4	483	10	3.5
Sept. 14	86	16	10.5	Oct. 5	274	23	5.1
Sept. 15	193	11	5.7	Oct. 6	241	20	8.7
Sept. 16	124	11	3.2	Oct. 7	157	36	9.6
Sept. 17	161	8	1.8	Oct. 8	101	54	16.0
Sept. 18	107	6	4.7	Oct. 9	139	35	10.5
Sept. 19	201	4	1.0	Oct. 10	92	45	13.0
Sept. 20	149	11	3.4	Oct. 11	76	51	3.9
Sept. 21	138	15	6.5	Oct. 12	77	32	5.2
Sept. 22	101	40	15.0	Oct. 13	96	28	7.3
Sept. 23	132	28	10.0	Oct. 14	79	24	7.6
Sept. 24	105	33	11.4	Oct. 15	44	29	2.3
Sept. 25	262	18	4.6	Oct. 16	36	33	25.0
Sept. 26	260	20	7.7	Oct. 17	39	49	10.3
Sept. 27	302	19	6.3	Oct. 18	27	48	0.0
Sept. 28	306	23	7.5	Oct. 19	14	57	7.1
Sept. 29	296	20	5.7	Oct. 20	27	15	3.7
Sept. 30	239	23	7.1	Oct. 21	24	17	12.5
Oct. 1	233	23	6.0	Oct. 22	11	27	0.0
Oct. 2	298	21	6.7	Oct. 23	13	23	0.0
Oct. 3	363	18	6.0	Oct. 24	40	30	5.0
Week ending-		Percentage of admissions developing pneumonia	Case fatality of pneumonia	Week ending-		Percentage of admissions developing pneumonia	Case fatality of pneumonia
Sept. 19		9.6	43.2	Oct. 10		24.14	30.64
Sept. 26		21.1	35.5	Oct. 17		34.23	22.22
Oct. 3		20.66	31.35	Oct. 24		30.13	14.90

It is seen here that as the outbreak progressed the proportion of cases complicated by pneumonia increased, while the case fatality of the same cases became less. This was not due to any change in treatment or to any discoverable change in the bacteriology of the cases. As the whole series was studied by the same men, it is unlikely that differing standards of diagnosis have any bearing, as might often be the case in comparing figures from different sources. Unfortunately, this camp is the only one presenting its figures in such a way as

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to allow this kind of a comparison, and it is not known whether this relation between pneumonia incidence and fatality holds generally or is only an accidental happening at Camp Upton.

The following account of the epidemic at Camp Grant, Ill., is introduced verbatim. Though Camp Grant stood fourth in the proportion of fatal cases during the epidemic, its proportion of admissions was not far above the average. It is thought that this account, written by the camp surgeon, will convey in a more satisfactory way than any other an idea of the character of this outbreak and of the difficulties besetting the course of the Medical Department in combating it.⁶⁸

CAMP GRANT DIVISION SURGEON'S REPORT^a

In the latter part of September a severe epidemic of clinical influenza attacked the camp, resulting in 10,713 cases of this disease during the months of September and October, with 2,355 cases of pneumonia and 1,060 deaths resulting.

The rapidity with which cases developed during the height of the epidemic promptly flooded the base hospital, and it became necessary to equip various infirmaries throughout the camp to receive patients. When the housing space in the infirmaries was filled, one or more contiguous barracks in each area were assigned for the reception of patients. All mild cases were received in the infirmary wards, and if the cases became more severe they were transferred at once to the base hospital. These wards were also used for the reception of convalescents returned from the base hospital, who were held for about a week for observation before being returned to duty. In addition to the observation and attention given the men in the infirmary wards by the surgeons of the various organizations, a medical officer of experience was detailed as visiting consultant. This officer visited each infirmary daily and gave his advice as to which cases should be transferred to the hospital.

The efficiency of the attention given in these wards is attested by the fact that although more than 2,000 cases were handled, but one death occurred in an infirmary and that man was a returned convalescent. As the number of convalescents multiplied, their care became a problem which was particularly acute on account of the lack of a detention or isolation camp. It was solved by granting furloughs to selected men after ascertaining that their families were able and willing to give them proper care or supervision.

As the infirmary wards began to fill up with convalescents rather than acute cases, it was noted that many of these men had pronounced tachycardia. A cardiovascular specialist was detailed to visit all these wards and report on all cases having heart symptoms, with recommendations. This officer examined all convalescents returning from furlough and made recommendations as to their disposition.

The character of the complicating pneumonia during this wave of the influenza cycle differed only in degree from those observed earlier; the atypical pneumonias of the earlier months became the rule now and, especially in the first few weeks of the outbreak, there were noted cases with pneumonic symptoms of a fulminating character lasting 24 to 48 hours and showing post mortem a characteristic wet hemorrhagic condition with little or no evidence of inflammatory reaction. As noted above, a few cases of this type appeared in the spring wave. They will be described in more detail in the consideration of the pathology and symptomatology of the epidemic. In the later weeks of the outbreaks these fulminant cases became more rare, the fatal cases were ill a longer period of time, and the clinical and pathological conditions observed corresponded more closely with those seen during the preceding winter and spring. Empyema, so common then, was rarely seen in the early weeks of the fall wave, but in the

^aSee also Chap. XIV, Vol. V, of this history for a description of this epidemic in the base hospital.

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period of decline of this wave it reappeared with increasing frequency. In general, the pneumonia of this period was regarded as bronchopneumonia, though many cases were recorded as of the lobar type. Possibly differences in diagnostic standards account for this, but in view of the figures already discussed, which show that the proportion of lobar pneumonia increases with the group susceptibility, it is probable that different groups actually showed a difference in the distribution of the two types. The epidemiologist at Camp Devens, Mass., commented at this time on the singular fact that while white soldiers dead of the disease characteristically showed the lesions of bronchopneumonia, colored soldiers succumbing at the same time were equally certain to show a typical lobar lesion.

The bacteriology of this outbreak or wave was extremely varied in different localities and even at any one station. This is later considered more in detail. A point which is of interest in this connection, however, as evidence of the greatly increased virulence of the primary virus at this time is the great prevalence of pneumonias associated with infection by the ordinarily harmless "mouth" type of pneumococci. Though in the earlier periods these types were found more frequently than in interepidemic periods, at the height of the pandemic they appeared to be almost the only representatives of the pneumococcus group to be found. The resistance of the patient was reduced to the point at which these relatively feeble invaders were able to infect.

As regards the different methods employed at different camps in dealing with the enormous numbers of sick during the epidemic period, some surgeons stressed the contagious character of the disease, and every suspicious case showing fever was at once admitted to hospital and isolated. With increasing numbers of cases, many of which developed pneumonia, this necessitated the opening of numbers of barrack buildings as annexes to the hospital, especially for the care of lighter cases and convalescents. Another plan was to admit to the hospital only the more severe cases, those showing more than an arbitrarily designated degree of fever, for instance, the less severe cases remaining under the care of regimental medical officers in infirmaries or in quarters. This plan had the advantage of relieving the hospitals to some extent but was open to the objection that cases of the diseases were scattered among the troops with resultant liability to further spread. At Camp Travis, Tex., a compromise method was adopted consisting in opening a large annex capable of caring for several thousand cases, under the care of a specially organized medical and nursing staff, the annex to be used for the isolation and care of uncomplicated cases, patients developing pneumonia being promptly transferred to the wards of the hospital proper.⁷⁰ Though this method would appear the most satisfactory on theoretical grounds, and Camp Travis showed a very low fatality with a large number of cases of pneumonia, consideration of the results in general shows no particular advantage in any of the methods of handling the situation. If, as suggested, later studies should show definitely that the infective period of the disease is mainly limited to the stage of invasion, the explanation of the failure of all these methods to reduce the incidence of the disease will be at hand.

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In correlating the epidemiological facts recorded above it may be said that they harmonize with the following conception of the epidemiology of influenza. First, the virus of the disease has a distinct tendency to periodic increases in invasive power, each such increase corresponding to a wave or outbreak of the disease. Between such waves the disease is not entirely absent, but attacks only sporadically. In addition to these changes in invasive power there are changes in the virulence of the infection, once invasion is accomplished. This fluctuation in virulence, if periodic, is of a much longer wave length than that of the fluctuations in invasiveness. The apex of such a wave of virulence corresponds to the outbreak of highest fatality in the course of the cycle. Though the virulence is measured by the proportion of cases developing secondary infections and by the fatality, the degree of invasiveness determines the number of cases attacked in a given outbreak. The disease itself is a comparatively mild one seldom fatal in the absence of definite secondary infection.

This conception would lead us to believe that the earlier waves of an influenza cycle would attack mainly those groups of individuals especially susceptible to invasion either by reason of individual characters, race, or previous environment. In other words, those who had little immunity, natural or acquired. Later waves of increasing invasive power would affect persons of greater resistance, each wave leaving its quota of immunes who are not attacked in later waves. When the invasive power is at its greatest degree of evolution persons most refractory are attacked.

Parallel to the increasing invasive power runs an increase in virulence as manifested by increasing proportions of complicating pneumonia and deaths. In the earlier waves the pneumonias found correspond more nearly to those seen in the absence of influenza, their fatality is less and the characteristic hemorrhagic lesion of the influenza lung is rarely seen. Each case is of longer duration and the spread of infection from the lungs resulting in empyema and other complications has time to manifest itself before the fatal issue. The anatomical type of pneumonia undergoes a change as more resistant groups are attacked, the lobar pneumonias becoming more rare and bronchopneumonia more frequent.

In the earlier waves, groups that are of known susceptibility to the disease were attacked in greater numbers; the fact that the figures for the whole war period show no such marked relation indicates that these groups of susceptibles were early attacked and to a large extent became immune, the less susceptible elements being affected later in greater proportion so that in the end there was little demonstrable difference in the incidence in the different groups.

After the peak of the outbreak of the cycle was passed the persistence of the infection was manifested for some months by the occasional occurrence of typical cases. When the next increase of invasive power manifested itself there was again a certain amount of susceptible material for the virus to attack, because of the short duration of the immunity conferred by the earlier waves, or, perhaps, the fact that possibly not all susceptible individuals, even during a great pandemic outbreak, come into effective contact with infective cases of the disease.

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ETIOLOGY

The experiences of the World War have tended to confuse rather than to clarify ideas as to the specific etiology of influenza and, indeed, of the pneumonias as well. Up to the occurrence of the 1918 epidemic the orthodox conception of the etiology of influenza was that of Pfeiffer, whose conclusions were based on work done late in the 1889-1892 cycle of the disease. With the advent of the earlier recognizable waves of the 1918 outbreak it became evident that the bacillus of Pfeiffer was not uniformly present in the cases examined. Certain laboratories reported a high percentage of positive results; others, apparently equally well qualified and equipped, failed entirely to isolate the organism or found it in only a small percentage of cases. Doubts arose as to the specific relation to the disease of the hemophilic bacillus, and it was recalled that this organism had been found frequently during the interepidemic period associated with other diseases-notably measles, whooping cough, and pulmonary tuberculosis-and indeed in persons apparently free from disease. A difference of opinion arose among bacteriologists which still obtains. The development and general use of special media favoring the growth of the Pfeiffer bacillus, such as heated blood agar, or media tending to inhibit growth of other interfering organisms, such media as Avery's oleate-hemoglobin, increased the number of positive findings. The general use of these media, however, came too late for study of the great autumn wave of the disease. It was found also that these media increased proportionately the number of positive findings among noninfluenzal individuals. The demonstration by Park and his associates that the various strains of B. influenz? isolated from epidemic cases in New York City were not serologically identical and indeed had little tendency even to fall into groups when subjected to agglutination and absorption tests with specific immune sera, was interpreted by many authorities to indicate that the hemophilic bacilli constitute a group of serologically heterogenous organisms rather than a single species, and that as an epidemic must necessarily be due to the same cause in any given outbreak, it is impossible to assign to the Pfeiffer bacillus the r?le of specific inciting cause of the epidemic.⁴⁹The prevailing attitude with reference to etiology is one of agnosticism, and until further work either produces conclusive evidence in favor of the Pfeiffer bacillus or presents some other organism for consideration as the cause of the disease, opinion on the subject will remain unsettled.

Various investigators have suggested a number of other organisms as the cause of influenza. The one which has received the most attention and which has been found in outbreaks not only in this country but also in Europe is a Gram-positive diplostreptococcus producing a green coloration on blood agar and seeming to partake of the characteristics of Group IV pneumococci and of hemolytic streptococci of the alpha type, if not actually a mixture of these organisms. All of the objections to the acceptance of the Pfeiffer bacillus as the cause of the disease apply with equal or greater force to this apparently heterogeneous group.

The bacteriology of the pneumonias throughout the course of the World War was also unsettling to preconceived ideas of the causation of the disease. At the outset the idea was generally accepted that the majority of cases of

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pneumonia were due to pneumococci of the so-called fixed types. It soon was proven, however, that not only were the pneumococci of the mouth types found in a much greater proportion of cases than was expected but also that other organisms, notably the hemolytic streptococcus, were associated with pneumonia in large numbers of cases. The technique of typing the pneumococcus was new to many laboratory workers, and the shortage of mice rendered it often necessary to fall back upon the blood-broth culture method of Avery, which has the disadvantage of failing to distinguish between virulent and avirulent organisms. However, the compilation of available figures from the months before the fall (1918) epidemic of influenza shows that 1,903 cases were reported from 29 different sources.⁷¹ These cases may be tabulated as follows:

Types of pneumococci reported from the camps in the months preceding the fall outbreak of influenza

Type	Cases	Percentage of total	Type	Cases	Percentage of total
I	349	18.2	III	106	5.5
II	278	14.6	Group IV	935	49.0
II (atypical)	235	12.7	Group IV	935	49.0

Inasmuch as most of these reports did not distinguish between the true and atypical types II it is probable that the proportion of the former should be considerably smaller and the latter correspondingly larger. About a third of the cases were charged to the strictly epidemic types of the pneumococcus. There were differences between camps in type distribution that can be accounted for neither by the care exercised nor by methods employed. It frequently happened that the same camp reported striking changes in the proportion of the types found from month to month.

As time went on and workers gained proficiency in the method of typing it was expected that the proportion of the fixed types would tend to increase. As a matter of fact the reverse was true. Reports on the types found during the months of September and October, 1918, show a still greater incidence of the mouth types of pneumococci.⁷¹ A tabulation of 2,617 cases from various sources is as follows:

	Cases	Percentage of total		Cases	Percentage of total
Type I	82	3.1	Type III	199	7.6
Type II	145	5.5	Group IV	2,090	79.9
Type II (atypical)	101	3.9	Group IV	2,090	79.9

Not only were the pneumococci of the heterogeneous group predominant but other organisms were encountered in association with them in various places and at different times during epidemics of pneumonia. Of these associated organisms, the most important was the hemolytic streptococcus. This organism appeared early in some camps as the exciting cause of postmeasles pneumonia, more fully treated elsewhere. Not all camps having a high incidence of pneu-

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monia after measles were able to attribute it to this organism. The facts with regard to the postmeasles pneumonia at Camp Wheeler have already been stated. Camp Travis, on the other hand, at the same time had a large proportion of these cases due to streptococci. It would appear that given the necessary predisposition in the form of primary infection, the actual exciting cause of the pneumonia will depend upon the predominance of one organism or another in the environment or upon the mucous membranes of the patient himself. This idea is supported by the fall outbreak in which most stations reported their pneumonia as of group IV pneumococcus origin but some attributed all their fatalities to the hemolytic streptococcus. During this time reports attributed pneumonia to still other of the usual flora of the mouth such as the streptococcus viridans, the staphylococcus (Camp Jackson),⁷²and the pneumobacillus of Friedlander (Walter Reed General Hospital).⁷³

The relation of pneumonia to the incidence of influenza has been shown. If, as seems certain, this is a true causative relationship, the influenza acting as the predisposing cause which rendered the pneumonic infection possible, it would appear that the earlier, less virulent waves of the epidemic were complicated by the more invasive of the pneumococcus types while, as the influenza itself became more virulent, it further reduced the resistance of its victims so that the ordinary mouth types were increasingly able to infect, and also such ordinarily inactive organisms as the staphylococcus, the hemophilic bacilli, etc. The hemolytic streptococcus, and organism of low invasive power combined with high fatality, invaded those camps first where for unknown reasons it happened to predominate. Later with the general interchange of troops between camps it became more widely disseminated. So far as the records show there is no instance of an outbreak of pneumonia due to this organism that was not coincident with an outbreak of influenza or of some acute respiratory disease that might readily be regarded as of influenzal nature.

It has proven impossible to estimate the number of cases of primary pneumonia that occurred in the Army during the war and it follows that no separate study of its etiology is possible.

PATHOLOGY

As was stated in the introductory paragraphs of this chapter, there is evidence in the findings at autopsy of patients dead of pneumonia early in the war period that some influence was at work tending to the production of findings at variance with the usual experience in the pneumonias of the interepidemic periods. That this influence was, at least in great part, the presence and activity of the increasingly virulent infective agent of influenza has been shown with some degree of probability. The nonfatal character of uncomplicated cases of influenza renders a description of the anatomic changes produced by that disease alone an impossibility. The autopsies were necessarily all held on the cases of men who had suffered fatal pneumonic complications. This situation renders it necessary to consider the subject of pathologic anatomy from the same standpoint as that of epidemiology. The pathology found in cases of influenzal pneumonia will be described and the attempt made to deduce the changes in uncomplicated influenza, at least of the more severe type, by first, noting those

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characteristics in which the pneumonias of the influenza period differ from those usually observed in the absence of an epidemic, and second, paying particular attention to the findings in those fulminant cases of very short duration in which secondary infection, if present, had apparently not had time to produce recognizable anatomic effects.

The sources of information drawn upon for this study are various. The unusual character of pneumonia very early led to careful studies on the part of hospital staffs, and to the publication of numerous reports. A special board of officers studied the condition at Camp Travis in the late winter of 1917-18, and later made observations at Camp Dodge, Iowa.⁷⁴Another board worked during the summer mainly at Camp Funston, Kans., and during the fall wave of the epidemic at Camp Pike, Ark.⁷⁵Special reports of the pathology during the last-mentioned outbreak have been published by officers at a large number of camps. An especially comprehensive study of the subject was made at New Haven in connection with the Army Laboratory School at Yale University.⁷⁶ Reports of the conditions found at autopsy in our forces in France were received from practically all large centers in response to a circular letter from the office of the director of laboratories and infectious diseases, dated December 17, 1918. Material from all these sources has been utilized in the attempt to draw a comprehensive picture of the changes found in these pneumonic cases, not only during the great fall epidemic but in the earlier months as well.

It is stated elsewhere that clinicians found nothing in the pneumonias of the months before late November, 1917, that struck them as being different from the findings of the interepidemic period in civil life. With the occurrence late in 1917 of the definite wave of acute minor respiratory diseases, which was in general coincident with the declining period of the great measles outbreak, the atypical forms of pneumonia began to make their appearance in increasing numbers. Cases in increasing proportion were recognized by the clinicians as bronchopneumonia rather than lobar pneumonia, and this proportion was still further increased by the results of post-mortem examination. Briefly, the findings at this period were characterized by their variability. In any given camp there was a tendency for one finding to predominate, but in no report is it shown that the pneumonias examined ran true to any one type in even the great majority of cases. The details found appeared to be somewhat dependent on the nature of the bacteria associated with the process. But there was everywhere considerable diversity in the bacteria found. Two outstanding facts were easily noted at this time: First, the fatal cases were characterized by a very large proportion of secondary or metastatic invasions of the serous membranes, particularly the pleur?, producing highly fatal empyemata; second, the high incidence of pneumonia was largely confined to camps which were made up of men from the States of the south and southeastern part of the country, States whose men had been shown by Civil War figures to be far more susceptible to the respiratory diseases in serious form than those of the north and west.

In more detail, the report of the commission at Camp Travis showed that anatomically the pneumonias might be classified into true lobar pneumonias associated with pneumococcus infection; interstitial bronchopneumonia

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in which the infection appeared to spread through the bronchial walls for some distance in the interstitial tissue of the lung, producing peribronchial nodular consolidation; lobular pneumonia, in which groups of lobules showed consolidation due to filling with polynuclear leucocytes; and mixed types showing two or more of the varieties described. The interstitial and lobular varieties were associated with infection by hemolytic streptococci and the lobar type usually with pneumococci. The influenza bacillus of Pfeiffer was found in a considerable proportion of cases but was not regarded as of primary importance.

Reports from Camp Funston at this time showed substantially the same conditions.⁷⁷In both camps (Travis and Funston), as indeed wherever fatal pneumonia prevailed, the incidence of suppurative complications, especially empyema and, to a less extent, peritonitis and pericarditis, was high and was responsible for a large proportion of the fatality. At Camp Funston in particular, a form of mediastinal abscess, "subcostosternal abscess" was noted post mortem with great frequency. These serous membrane suppurations were commonly of streptococcus origin. They occurred, however, in pure pneumococcus infections, though with less frequency. In general it appears possible to make the statement that at this time, of the camps showing a high incidence of pneumonia, those with the greatest fatality showed the highest proportion of the lobar type of the disease.

Reports of the pneumonias occurring with the spring (1918) outbreak of influenza showed an increasing proportion of the types of bronchopneumonia, though typical lobar pneumonia still appeared. At this time the number and character of influenza cases forced attention to this disease at most camps. On the other hand, at Camp Dodge, Iowa, for instance, while the medical authorities there reported the coincidence of large numbers of cases of mild tracheitis,⁷⁸McCallum,⁷⁹who studied the outbreak, stated that the majority of the cases could not be shown to be related to any recognizable previous disease. This shows that even when looking for such an occurrence it is possible to fail to recognize the existence of the primary infection. That the reported cases of mild tracheitis at Camp Dodge were in fact influenza, and the pneumonias of that period were influenza pneumonias will hardly be questioned at this time in view of the experience of other camps during the same months and the subsequent evolution of the pandemic. If more definite proof were needed, it may be found in the description of the pathology of the pneumonias studied. In general the findings parallel those of the previous winter at Camp Travis. Certain changes are noted that appear very significant in the light of later events. The proportion of lobar pneumonias was smaller, as was that of the interstitial type of bronchopneumonia. Lobular pneumonia predominated, often confluent. Serous membrane infections, while frequent, were present in a smaller proportion of autopsies, as patients were more apt to die before these complications had time to develop. "At necropsy there was found the most intense congestion of the entire respiratory tract." The bronchi "were markedly dilated toward the periphery; their mucosa was deep purplish gray." "Great hyperemia and edema of the bronchial wall is seen accompanied by a less evident new formation of connective

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tissue cells. The adjacent alveoli so far as they are not filled with red blood corpuscles, are packed with mononuclear cells and dense fibrin. The alveolar walls are infiltrated with mononuclear cells and are widened." Large necrotic areas were noted, eventuating in abscess formation and containing enormous numbers of streptococci. Perhaps most interesting in evidence of the influenzal character of the disease was the occurrence of the hyaline change in the lower third of the rectus abdominis (Zenker's degeneration) in two cases with rupture of the muscle and hematoma. All these points are so characteristic of the pneumonias of the fall outbreak, unquestionably influenzal in nature, that their occurrence at this time in the presence of the epidemic of "mild tracheitis" serves to clear up the character of that infection.

With the passing of this wave of infection the type of pneumonia recorded was predominantly lobar until the advent of the highly fatal wave in September. The following brief account of the findings in the latter wave, during which the influenza manifested itself at the height of its virulence, is necessarily a composite, culled from many published and unpublished articles and reports. In the interpretation of the findings with regard to the part played by the influenza virus in the production of the picture as well as that played by secondary invaders it is necessary to consider in particular two factors with regard to any given case. These are the duration of the disease before the fatal ending and the stage of the epidemic wave at which the onset of the disease occurred. In the consideration of the varied pathology of the epidemic it is necessary to divide the cases into certain groups which are fairly difinitely correlated with the variations in the two factors mentioned.

The first group of cases showing best, post mortem, the lesions which will be interpreted as characteristic of the primary influenzal infection, has been called the "wet, red lung" type, or hemorrhagic pneumonitis. It occurred typically in the earlier part of any given outbreak, and then usually in cases of less than average duration, often of only two or three days. A second group, comprising a majority of deaths during an epidemic, showed various pneumonic lesions on a hemorrhagic background. These occurred in cases of somewhat less initial severity than those of the first group and life was prolonged sufficiently to allow the lesions of secondary infection to fully develop. These cases was found in greatest proportion during the height of the outbreak and showed an average duration decidedly longer than those of the purely hemorrhagic type, even as much as ten days or two weeks. The type of consolidation encountered in this group varied greatly according to the location of the troops involved, according to race, and possibly to other factors. Extrapulmonary suppuration, such as empyema, was rare, although fibrinous pleuritis or pleuritic effusion were often reported. Still later, in an outbreak, a third type of cases came to autopsy. These were usually cases of still longer duration and in them the hemorrhagic features were much less noticeable. So also the acute pneumonic consolidations were not so constant, but evidence of their former presence was at hand in the shape of unresolved pneumonia, organization of exudate, pulmonary suppuration, interstitial or in the form of abscess, and secondary suppuration of the serous membranes. This last group then resembled most closely those of the previous winter, those of the second group the more acute cases of

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the spring outbreak, as described by McCallum,⁸⁴at Camp Dodge, while the hemorrhagic group was represented in the spring outbreak by the rare cases of the type already mentioned. These different types must be the result of the interaction of the virulence of the virus on the one hand and the degree of resistance of the persons attacked on the other. As both factors are variables all types of pneumonia were produced during the evolution of the influenza cycle and in any one outbreak graduations from one type to another were encountered and mixed types were frequent. Bearing in mind the above facts of distribution we may continue to the more detailed description of the types of pulmonary lesion found.

The wet red lung or hemorrhagic pneumonitis gave a picture almost pathognomonic of acute influenzal pneumonia. The only comparable findings are those of pneumonic plague and those seen in acute death from toxic gas. On opening the thorax the first point noted was the almost total failure of the lungs to collapse. On removal the lung retained on the table its natural size and shape. The pleur? usually contained a little blood-tinged fluid, rarely any considerable amount. The pleural surface was usually smooth and glistening, though a thin layer of fibrin over areas of greater density was not rare. The pleural surface of the lung was brilliantly mottled throughout with different shades of red, from the pale pink of emphysema through the bright color of recent hemorrhage to the deep purple of venous blood. This mottling was lobular in distribution, contrasting colors often showing in adjacent lobules. There were usually considerable areas of definite emphysema, especially along the anterior borders. The posterior portions, especially of the lower lobes, showed the darkest coloration, but the changes described usually involved all the lobes to some extent.

On section of such a lung, large quantities of bloody serum escaped from the cut surface, usually containing small bubbles of air. It seemed impossible to dry the surface by scraping. Portions cut from the lung usually barely floated in water; some sank. There was seldom any distinct nodular feel to the lung, though some portions were evidently more solid than others and the cut surface of such areas presented a meaty appearance very similar to that of a recent hemorrhagic infarct. The entire tracheobronchial tree was intensely congested, of a deep velvety red, spotted here and there with foci of a darker or more intensely red color. The bronchi contained a thin seromucous blood-stained fluid and the bronchioles, especially peripherally, were distended and prominent. Rupture of these distended bronchioles was associated occasionally with interstitial emphysema, usually confined to the lung, but more rarely becoming generalized subcutaneously. The extent to which the lung was involved in this hemorrhagic and edematous process was at times incredible and was plainly of itself incompatible with life. The condition was characteristically diffuse involving as a rule all the lobes to a greater or lesser extent.

Histologically the tracheobronchial lesions were prominent. In the trachea the mucosa was always to some extent destroyed, and large areas were denuded of epithelium. Such areas were covered by an exudate composed of red blood cells, mucus, and small amounts of fibrin. The exudate was rarely adherent or diptheritic in character. The submucosa showed a marked degree of edema,

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congestion, or even rupture of blood vessels with hemorrhage. In the earlier cases at least there is no sign of leucocytic reaction. The same general findings apply to the bronchi and bronchioles to an even greater extent. The epithelium becomes hyaline, loses its nuclei, and is frequently desquamated. The vessels of the submucosa are dilated, project into the lumen, and often rupture. The muscularis may be involved with loss of nuclei and hyaline change. (It seems probable on clinical grounds that in many cases the disease may be limited to the trachea and bronchi so far as pulmonary extension is concerned.) Wherever sections of the lung are taken for examination the conditions found are much the same. There is a general edema. Subpleural spaces, interstitial tissue, alveolar walls, and alveoli are all involved in this process. The characters of the exudate are best seen in the alveoli where it sometimes appears as a homogeneous mass resembling colloid, sometimes as finely granular material, and sometimes containing strands of fibrin. Varying numbers of red blood cells are present in this fluid from a scattered few to densely packed masses indistinguishable from a recent infarct. In some cases large numbers of bacteria are found throughout; in others careful search fails to reveal their presence. In these early cases polynuclear leucocytes are conspicuously absent. Slightly later, lymphocytes and large mononuclear cells are prominent in certain areas.

Winternitz and his coworkers state that⁷⁶ "The acute death which involves the tracheal, bronchial, and bronchiolar epithelium and which may extend beyond the epithelium into the walls of these structures and kill en masse the walls of the alveoli is a lesion which does not occur in other types of pulmonary infection." In influenza it is the lesion of characterization. In addition the aplastic reaction of the lung characterized by edema, hemorrhage, and lack of leucocytic response produce a pathologic picture in these fulminant cases that will hardly be confused with that of any other disease. Certain details have been emphasized by different students of the condition which deserve mention. Le Count⁸⁰ called attention to necrosis of the walls of the pulmonary capillaries. Wolbach⁸¹ regarded a peculiar distribution of a hyaline fibrinlike substance on the walls of distended alveoli as characteristic of the disease. Most reports mention a tendency to hyaline thrombi in lymphatics and in smaller arteries and veins. The broad general characterization of the process, however, as showing evidence of a toxic injury of the tracheobronchial epithelium, the process extending entirely through the walls of these structures and accompanied by an aplastic inflammation of the pulmonary parenchyma with marked generalized edema and hemorrhage, suffices to define the findings.

The findings in later cases show various types of pneumonic consolidation on a background of varying degrees of the condition just described. In these cases the power of reaction on the part of the tissues appears to have been at least partially restored. Lobar, interstitial, and lobular forms of pneumonia, with exudation of polynuclear leucocytes and often fibrin into the consolidated areas, characterized this group, the appearances differing from those found in the earlier months by reason of the greater prominence of the hemorrhagic and edematous background upon which the inflammatory process was superimposed. The existence of the true lobar type of consolidation during this epidemic has been questioned by many but the following statement on the subject from the report of the Camp Pike commission should prove conclusive:¹³

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The pulmonary lesion has been designated lobar pneumonia when it exhibited the well-known characters of this lesion, namely, firm consolidation of large parts of lobes, coarse granulation of the cut surface, fibrinous plugs in the bronchi, and on microscopic examination homogenous consolidation and fibrinous plugs within the alveoli.

This commission found lobar pneumonia in almost half their autopsies. Everywhere various combinations of the different types of pneumonia were found and in different localities different types predominated. It would appear that, in general, the lobar type of the disease was associated with pneumococci, the interstitial type with streptococci, the lobular type with streptococci or the Pfeiffer bacillus, and a type characterized by small peripheral abscesses with the staphylococcus. Exceptions to these rules are so numerous, however, that definite conclusions may not be drawn from the anatomical findings. Pneumococci may be responsible for either lobar or bronchopneumonia. An especially interesting point, perhaps bearing on the pathogenesis of these conditions, is found in the report of the Camp Pike commission.¹³ Cultures, post-mortem, were taken from the bronchi, lungs, and heart's blood. In cases of bronchopneumonia of pneumococcus origin the percentage of positive findings in the bronchi considerably exceeded that in the blood while in pneumonias of lobar type the reverse was true. This may be evidence of a different route of infection in the two types.

A detailed description of the various lesions observed in these pneumonias is not in place here. Suffice it to say that as the cases occurred later and later in the epidemic their type more and more nearly approximated those of the earlier months, showing increased pneumonic consolidation and less prominence of the hemorrhagic and edematous condition. Secondary suppurations became more frequent. The same characters were found in the pneumonias of the recurrent waves, including that of the early months of 1920.

It appears impossible to attempt a statistical analysis of the variations of pathologic type between different stations or geographical divisions. The impression is gained from the study of numerous reports that the lobar type of the disease was more generally reported from the southern camps than from the northern. The reports of the hospital centers in France while showing the same diversity of findings, indicate the relative infrequency of lobar pneumonia among the troops in that country. The reports seldom divide their cases according to race, but that the same preponderance of lobar pneumonia in the negro that is noted in the admission and death rates also held in the postmortem room is suggested by the comment of the epidemiologist at Camp Devens already quoted.

While, post mortem, the pulmonary lesions assume commanding importance, from the epidemiological standpoint, the lesions of the upper respiratory tract are perhaps of greater significance. The nearly constant pharyngitis is emphasized clinically. Involvement of the accessory sinuses of the nose, especially the sphenoidal, was reported with great regularity by pathologists who looked for the condition. The early conjunctival inflammation observed clinically is perhaps related to the upper respiratory tract infection or may possibly represent the atrium of infection.

Of changes outside the respiratory tract, few appear to be of significance in influencing the outcome of the disease. A possible exception is the lesion

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in the adrenal recorded in several reports. This appears to have been of a hemorrhagic nature, with loss of the lipoidal contents of the parenchyma cells. Some observers have connected this lesion with the prostration so characteristic of the disease. Of other changes, particularly emphasized in the reports, the occasional occurrence of interstitial emphysema, in some cases involving a considerable part of the body surface, and the hyaline degeneration rather commonly observed in the lower third of the rectus muscle with frequent rupture and hemorrhage, have already been dwelt upon. The degenerations of the parenchymatous organs, though often extreme were in general those seen with any severe toxemia, although focal hemorrhage apparently due to lesions of the vessel walls were not uncommon. Of suppurative complications, seen mainly in cases of relatively long duration, empyema, peritonitis, pericarditis, and meningitis occurred with varying frequency.

ROENTGENOLOGY

The result of systematic examination of cases of influenza and pneumonia by means of the X ray has been exceedingly valuable, not only in enabling the clinician to greatly increase his knowledge of the progress of the disease in the individual patient, but also in throwing much light on the nature of the early changes taking place in the lungs, especially in cases of uncomplicated influenza in which other methods of exploration of the chest are impossible. Thus the subject of roentgenology might be considered appropriately either under the heading of clinical medicine or under that of pathology. It seems best to consider the X-ray findings themselves with occasional comments as to the interpretation that seems justified or the special clinical value of the observations.

One of the most important questions put to the Roentgenologist for answer was whether or not early uncomplicated cases of influenza show any demonstrable pulmonary changes. The majority of cases give no evidence of such involvement on physical examination, or at most a few scattered r?les usually interpreted as bronchitis. The important question was: Does the generalized hemorrhagic and edematous condition found post mortem in fatal cases exist in uncomplicated influenza cases without pneumonic consolidation; and if so, to what degree? The answer to this question is important in the interpretation of the post-mortem pathology, in the study of the pathogenesis of the complicating pneumonias, and consequently in the development of means of preventing these complications. In several hospitals careful X-ray studies were made and reported, which appear to answer this question quite definitely. The method adopted was to take daily, serial plates from a large number of cases from admission to recovery or death. In some cases control plates, taken previous to the onset of the disease, happened to be available. The results of these studies on cases, clinically without signs or symptoms of pneumonia, are summarized in the following quotation from the report of two investigators at Walter Reed General Hospital:⁸²

On examination of the radiograms there is seen to be a general increase of density throughout the lungs. This density may be described as being of a hazy or smoky nature, which decreases the contrast between the normal lung transparency and the bronchial tree outline. This generalized density is somewhat greater from apex to base, to the inner half

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and especially adjacent to the mediastinal border, thus obscuring to some extent the mediastinum and cardiac outline. The most marked changes are seen in the bronchial structures themselves. A greater number of vessels are seen than is usual in any other disease. They are more definite, although diffuse in outline. This peribronchial thickening, however, is seen most markedly around the hilius, extending outward in "sunburst type" and rapidly diminishing in size from the hilus to about the outer one-third of the lung. The peribronchial thickening extends upward parallel to the mediastinum and helps to obscure the mediastinal outline. The greatest thickening and the greatest diffuse bronchial density are seen at the base, extending downward from the hilus, reaching the diaphragm outline and extending to just beyond the midclavicular line.

In cases where influenza progressed as influenza toward recovery there is a gradual diminution, first, in the hazy generalized density and then in the diffuse density of the bronchial structure. The density around the hilus due partly to peribronchial thickening remained considerably longer. The hilus becomes affected very early. There is an immediate enlargement of the hilus and marked increase of density with irregular outline, so much so that in no case is there any question of the reaction of the hilus. The irregular outline is due to bronchial thickening.

The conditions outlined above obtained in the case of average severity, without signs of pneumonia. When present in slight degree, as in unilateral cases, or in cases showing a little more than the enlargement of the hilus structures, the clinical severity of the case was invariably slight and the return to normal was rapid.

Certain cases of average severity, however, showed on physical examination persistent r?les at either base. At Fort Sam Houston, Tex., 25 per cent of 1,000 cases of uncomplicated influenza gave this finding.⁸³There was no other evidence clinically of anything resembling pneumonia; however, X-ray plates showed local areas of increased density corresponding to the physical signs in practically all of this group. This finding is probably to be interpreted as an early or slight example of the lesion described from Walter Reed General Hospital as "hemorrhagic pneumonitis." This condition was correlated definitely with the hemorrhagic infarctlike consolidations discovered post mortem. It showed on the plates as a denser, smoky opacity obscuring the bronchial outlines, beginning at the hilus and extending peripherally. It may involve all the lobes, but is seen most often in the lower. This shadow differs from the shadows of pneumonic consolidations both in distribution and in lesser density. Unless of considerable extent this condition gives no physical signs other than the persistent r?les mentioned; however, when its increase in extent reaches the surface, the signs produced are those heard early in pneumonic involvement and before classical signs of consolidation are established. Not all cases showing this type of consolidation eventuate in definite pneumonia, but in cases showing such involvement of a large proportion of pulmonary tissue the usual outcome is pneumonia, or, in the most severe cases, death, with lungs showing only the hemorrhagic and edematous type of lesion. The X-ray evidence, then, shows that some degree of pulmonary involvement is present in practically all cases of influenza, evidenced by a hazy opacity generalized throughout the lung, and a thickening of the bronchial structures and increased density of their shadows. This finding corresponds well with the generalized hemorrhagic and edematous lesion of the bronchi and parenchyma described on post-mortem examination for cases dying early, and justifies the belief that substantially this condition in varying degree exists in milder cases that recover without complication.

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With the advent of definite secondary infection the plates show a great variety of pictures. Briefly, the bronchopneumonic consolidations tend to begin near the hilus, to show first as a dense peribronchial thickening, with later nodular shadows grouped along the bronchi in such a manner as to suggest the "pussy willow." Still greater extension results in confluence and the production of massive shadows resembling those of lobar pneumonia. When resolution occurs it progresses from the periphery toward the hilus, in inverse order of occurrence. Lobar consolidation shows almost simultaneously throughout the area involved, with a tendency to peripheral involvement first and resolution is first central, the peripheral portions retaining the shadow to the last. Interlobar pleuritis is frequently demonstrated by the X ray when no physical signs are to be discovered. It shows as a fine almost hairlike line in the anatomic location of the fissure involved. Pleuritic fluid and empyema give the well-recognized pictures of these conditions.

Daily X-ray examinations show that the consolidations, especially those of the purely hemorrhagic type, vary greatly in extent from day to day, spreading and receding to spread again sometimes over the same area as before. These advances and recessions of the anatomic process are usually well correlated with the temperature and other clinical evidences of the intensity of the disease process.

In general it may be said that the X-ray plate often, if not usually, furnishes the first indication of the onset of complicating pneumonia. With resolution, the final clearing of the lungs from the X-ray and clinical standpoint is often not simultaneous. In some instances when the physical signs persist after the plate shows return to normal, the patient is said to feel and appear perfectly well. In the other group where the X ray shows pathology after physical signs are negative the patient has rarely completely recovered from the symptomatic standpoint. It would appear from this that the radiographic evidence of recovery was the more dependable.

CERTAIN CLINICAL ASPECTS

In the consideration of the clinical aspects of the respiratory group of diseases dealt with herein, only such features are touched upon as represent advances in knowledge during the World War, or as are of especial importance in the recognition or treatment of the conditions described.

A knowledge of the exact limits of the period of incubation is not easy to obtain, owing to the great prevalence of the disease when present. That the period is very short is generally conceded; that it may be of several days' duration, however, seems probable from certain reports, especially those which mention a period of malaise preceding the actual rise of temperature.

The classical descriptions of influenza divide the cases into various types, respiratory, gastrointestinal, meningeal, septicemic, etc. The cases as they presented themselves during the cycle of 1918 were remarkable for their uniformity of type. Minor differences in symtomatology are noted in the descriptions published from different sources but there is general agreement that the respiratory type of the disease predominated greatly. It would appear that, for this pandemic at least, influenza appeared as primarily involving the respira-

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tory tract and that symptoms referable to other organs were not only rare, but seemed due either to toxemia or to extension of secondary infection. Whereever a definite outbreak occurred the cases were so strikingly similar in their clinical manifestations as to leave no doubt as to their essential unity. During the severe outbreak there was a gradual change in the course of the cases, later cases running a more prolonged course with a greater tendency to the development of leucocytosis, and of secondary suppurations.

Though the clinical descriptions were in general very closely followed in the uncomplicated cases, some unusual complications were relatively frequent. These latter are considered below. One point that appears important in diagnosis is the very general agreement that the throat presented a characteristic appearance at onset. The various descriptions agree closely that the characteristic influenza throat showed a brilliantly red, glazed appearance of the pharynx and fauces without swelling or exudation. This appearance was most marked on the soft palate, and the sharp delimitation of the reddening at the margin of the hard palate was stressed. This throat condition was described not only in connection with the spring outbreak but with that of the fall as well. All patients complained of some degree of sore throat.

During severe outbreaks, many patients exhibited a hemorrhagic tendency. The frequency of this condition appears to have varied somewhat with locality. It was more common in the Army in the United States than in the American Expeditionary Forces. It was most commonly manifested by epistaxis, often recurrent, but hemmorhages from other mucous surfaces were not rare. Early and profound cyanosis of a peculiar color, described by some as "heliotrope," occurred in the more severe infections, especially in fulminant cases and in those that later developed pneumonia. Extreme prostration, out of proportion to the degree of fever or the duration of the illness, was generally recognized. So, too, was the slow return to normal in convalescence.

All degrees of severity of the primary infection were noted. At one extreme were the fulminant cases, resulting in death so promptly that secondary infection could hardly have had time to develop. At the other extreme were cases occurring in considerable numbers during an outbreak, and noted especially in the hospital personnel, in which the marked malaise and slight sore throat, with or without a little rise of temperature, were not regarded by the individual attacked as of sufficient importance to warrant relief from duty.

It has been recognized for years that the white blood count in influenza showed no hyperleucocytosis. This was confirmed during the pandemic under consideration. Extremely low counts were found at the time of greatest virulence of the infection. In the fall of 1918, almost any large series of counts showed a few as low as 2,000 or even lower. There is some evidence tending to indicate that counts lower than usual were of relatively bad prognostic significance. The following figures from Camp Custer, Mich., indicate that cases later developing pneumonia showed low counts on admission in greater proportion than did uncomplicated cases.⁸⁴

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Leucocyte counts in influenza, Camp Custer, Mich., September-October, 1918

White blood cells	Uncomplicated cases	Cases developing pneumonia	White blood cells	Uncomplicated cases	Cases developing pneumonia
	Per cent	Per cent		Per cent	Per cent
4,000-6,000	16.5	28.8	12,000-16,000	8.3	8.3
6,000-8,000	52.7	39.0	16,000-20,000	2.5	2.9
8,000-12,000	19.5	17.7	Over 20,000	0.5	3.3

In both series the mode falls in the 6,000-8,000 group. This coincides with the usual experience. Of all cases at Camp Jackson, S. C., 42.2 per cent had counts of less than 5,000, with an average of 6,344.⁷² This included complicated as well as uncomplicated cases. The average of cases that developed pneumonia was 7,141. A report from Camp Hancock, Ga., at the time of the spring epidemic summarizes the results of 202 counts as follows:⁸⁵

Leucocyte counts in influenza, Camp Hancock, Ga., April, 1918

Day of disease	Average count	Day of disease	Average count
1	6,166	4	8,158
2	5,378	5	8,959
3	7,522	6	7,855

The results of differential counting of the leucocytes are also concordant in the different reports. There is general agreement that the polynuclear elements are relatively reduced and the percentage of lymphocytes increased. The lymphocyte percentage during the spring epidemic often exceeded 50. In the later days of the outbreak this disproportion was not so marked, but still remained evident. Experience in the severe fall outbreak did not show so high a percentage of lymphocytes. Thus from Camp Jackson, S. C., it was reported that there was an average of 35.5 in uncomplicated cases, 27.8 in those that developed pneumonia.⁷²The opinion was generally expressed that there was no relative increase of polynuclear elements unless secondary infection impended. In general the statement is almost universal that the differential count was normal or showed a relative lymphocytosis.

The clinical types of pneumonia occurring during the 1918 cycle of influenza differed so decidedly from those usually seen in interepidemic periods and even from those described for previous pandemics that some description of these cases seems necessary here. A general view of the pneumonia situation during the World War shows that there was a gradual change in the predominating clinical type of the disease from the early cases, which were in no way noticeable as different from the usual type of pneumonia, to the very atypical pneumonias of September and October, followed by a distinct tendency to revert to the types of earlier months as the epidemic declined. This change was manifested in the relative proportions of lobar and of bronchopneumonia, in changes in the proportion of cases developing secondary suppurations, notably empyema.

As stated above, prior to December, 1917, cases of pneumonia, either primary or complicating measles, were regarded by our medical officers as differing in no essential from such cases occurring previous to the World War.

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Pneumonia complicating measles was not of unusually frequent occurrence. Particularly was this true as regards the earlier weeks of the great measles epidemic in the camps. However, by December, 1917, when there was a sudden increase in incidence of mild respiratory infections, variously recorded as influenza, bronchitis, coryza, etc., the number of cases of pneumonia apparently primary, as well as cases associated with measles, rose suddenly to alarming proportions especially in some of the southern camps. The pneumonia of this period was carefully studied by a special commission of medical officers working at Camp Travis, Tex.²They noted the presence at the time of an "epidemic of coryza, laryngitis, and mild bronchitis" in both the civil and military population of San Antonio. The report of the commission states that clinically the pneumonias studied could be divided into three groups: Bronchopneumonia, associated as a rule but not invariably with a recent measles attack; (2) lobar pneumonia, giving the familiar signs and symptoms of that disease and sometimes following measles; (3) a group showing clinically and anatomically a combination of groups 1 and 2. The description of the bronchopneumonia found there is of especial interest not only because it was the unusual feature of the outbreak, but because it is of value in comparing it with the bronchopneumonia of later waves. The cases at San Antonio were nearly all associated with the hemolytic streptococcus, though nearly half of them showed the presence of the Pfeiffer bacillus as well. Onset was gradual and without definite chill or sudden elevation of temperature, whether occurring during the course of measles or only after an interval of several weeks. The temperature rarely exceeded 104? F., and was frequently irregular even in the absence of empyema. The pulse rate was not extremely rapid even in cases near death. Respiration, too, was not extremely rapid but was characterized by extreme respiratory difficulty. Cyanosis was constant even in early cases.^a The cough was troublesome and the sputum varied in character, though not showing the tenacious rusty type, typical of lobar pneumonia. Pain was usually marked and was associated with the frequency of pleural infection. When noted, consolidation was usually at the base. R?les, musical, squeaking, or moist, were usually heard throughout the chest. In some cases characteristic signs of consolidation in a certain area persisted for a few days and then entirely disappeared. In uncomplicated bronchopneumonia, wide areas of dullness and tubular breathing were never observed. When such signs were found there was invariably a concurrent lobar pneumonia. Empyema complicated about one-half of the cases studied and its fatality was at least 50 per cent. The lobar pneumonia studied showed the presence of pneumococci, the epidemic types being demonstrated in two-thirds of the cases. In the series showing combined lesions both pneumococci and streptococci usually were demonstrated.

During the 1918 spring epidemic, pneumonia was for the firsttime attributed to antecedent influenza in any considerable number of cases,although the diagnosis previously had been made. The clinical types of pneumoniaseen in March and April corresponded well with those seen in the camps show-

^aOther observers have stated that cyanosis on admission for measles characterized cases that developed pneumonia later.

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ing a high death rate during the winter months. At this time the camps were much more generally affected. Empyema was still common and the mortality was generally associated with this complication. A small number of cases of a new type of the disease was seen for the first time during this outbreak. A patient with an attack of typical influenza of two or three days' duration would, after a day or two of normal temperature, develop acute pneumonic symptoms and die within 48 hours. This was the fulminant type of influenzal pneumonia familiar in the fall outbreak. The proportion of cases recorded as lobar in type was lower in the spring than in the winter and the case fatality of pneumonia was higher.^78,⁷⁹

During the period of lower incidence of respiratory infections following the spring outbreak and lasting throughout the summer, this increased case fatality of pneumonia cases persisted and even increased as is seen in the monthly tables. The increase culminated in the month of September, 1918, with the violent outbreak of the most severe influenza wave, which first showed its great virulence in the northeastern camps.

The percentage of influenza cases developing pneumonia at this time varied in different localities. The maximum figures were about 25 per cent. The usual case fatality at this time was around 30 per cent. Though the disease, as seen in different camps, varied somewhat in its clinical manifestations due to the various factors that have been discussed, the general characteristics of the complication were very constant. Few observers were able to distinguish clinically with any definiteness in the early stages of the disease between cases which showed later lobar lesions and cases of bronchopneumonia. The following condensed description of influenzal pneumonia of the most fatal type is drawn largely from a series of studies made at the WalterReed General Hospital, D. C.^82,86

The onset of the pneumonic complication occurred either after two or three days of normal temperature following an attack of influenza, or it developed gradually without there being an afebrile interval. In the former group the onset was often characterized by chill and sudden rise of temperature. The severity of the disease was correlated with the amount of lung involvement, unilateral cases doing much better than those with both lungs affected. Fulminant cases with severe toxemia showed rapid involvement of the entire lung.

In nonfatal cases, usually presenting a unilateral lesion, the temperature ranged from 100? to 103? F. The pulse was characteristically slow; the blood pressure low, the systolic figure often below 100 mm.; respiration was only slightly accelerated. Nonfatal cases usually recovered after an illness of about a week and defervescence was by crisis in some series, by lysis in others. In cases with bilateral lesions the cyanosis was more marked, even to an indigo blue color, the temperature ranged somewhat higher than in the unilateral cases and often showed variations paralleling the advance and recession of the pulmonary lesion as shown by the X ray or by physical signs. Cough was frequent and exhausting; the sputum, blood tinged or mucopurulent. In the more toxic cases, terminating fatally, the color of the patient from the first was either that of an intense cyanosis or a muddy, claylike pallor. The pallor was of particularly bad prognostic import. Nervous symptoms appeared early, rest-

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lessness and delirium being marked. The respiration became very rapid and dyspnea was pronounced. Physical signs of irregular consolidation and of edema filled the entire chest. The temperature ranged to 105? F. or higher, and death occurred in from three days to a week. It is evident that these groups were not clean-cut and that all degrees of varying severity intervened. Inasmuch as such a proportion of severe pneumonia has in the past seldom been associated with influenza, it is important to record in somewhat greater detail the peculiarities of this outbreak.

The first point to strike the observer was the universal occurrence of cyanosis. This condition appearing in an apparently uncomplicated case of influenza, if of a degree at all marked, usually presaged the onset of pulmonary inflammation. Whether due to toxic changes in the composition of the blood or to mechanical interference with oxygenation by the exudate in the lungs, the intensity of the cyanosis was, in general, an index to the severity of the case. In milder cases of influenza, a peculiar shade of "pink cyanosis" was observed, an erythematous flush of an unusual shade. The well-established case of pneumonia showed a shade that was usually described as heliotrope, and in the most asthenic group, usually associated with coma vigil, a muddy clay-colored pallor prevailed.

In some series of cases the tendency to hemorrhages from the mucous membranes was very notable. Epistaxis, which occurred in 10 per cent or more of the cases, was of all degrees, but often severe, recurrent, and debilitating in the extreme. Purpura, intestinal, and renal hemorrhages also occurred.

Of respiratory symptoms proper it may be said that these differed relatively little from the respiratory symptoms of the usual pneumonias. Pleuritic pain was frequent, cough was distressing, and frequently there was so much expectoration as to make resorting to narcotic relief seem dangerous. The character of the sputum varied from the tenacious rusty expectoration of typical lobar pneumonia, through varying degrees of mucopus, and frothy blood-stained material to the profuse pink froth in the mouth and nose which characterized the fulminant cases. The typical rusty sputum was rare, but the presence of some amount of blood was the rule.

From the beginning the physical signs were confusing. Typical signs of consolidation were seldom found, and then late. After some experience with these cases most observers concluded that the diagnosis of pulmonary involvement was better made from the general course and symptomatology than from physical signs. Here, too, the X-ray examination proved very valuable, as was stated above. The early signs of pneumonia were confined to the presence of fine scattered r?les, and as these r?les were found in many apparently uncomplicated influenza cases their significance was not clear. As the involvement proceeded, dullness became evident on percussion, and breath sounds, voice, and fremitus were diminished, thus suggesting fluid in the pleura. Areas of tympany were also observed. After several days the confluence, or extension of consolidated areas, often produced typical signs of consolidation. Pleuritic friction was often heard.

The heart action was slow in proportion to the temperature, and right-sided dilatation was not the rule even in severe and fatal cases. Low blood

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pressure was noted, in some cases the systolic blood pressure falling as low as 80 mm. without a necessarily fatal issue. The temperature was very variable, usually of a fairly continous type, but in some cases remissions with sweating were frequent even without suppurative complications. The leucocyte counts were also variable, some fatal cases showing no change from the initial leucopenia. In others a marked polynucleosis supervened. Pneumococcus cases showed this rise earlier than did cases infected with streptococci. Blood cultures were positive in a relatively small proportion of cases, and pneumococcus infection gave the great majority of the positive results.

Toxic nephritis, varying in degree, occurred in nearly every case. The presence of large numbers of casts was almost invariably of fatal import. Gastrointestinal symptoms were rare, though early and persistent vomiting occurred in the highly toxic cases. Constipation was the rule. Toxic involvement of the nervous system was evident in all the more severe cases. There was sleeplessness, restlessness, severe headache and, to a greater or lesser degree, delirium. The delirium appeared to be related to the degree of toxemia rather than to the temperature. Two types of delirium were noted: A restless talkative type, hard to control, but unassociated with a lack of orientation when the patient was questioned; the coma vigil type. The talkative type was not of very bad prognostic import, but almost all cases who had the coma vigil type of delirium died.

Skin eruptions were prominent in some series of cases and hardly mentioned in others. At the Walter Reed General Hospital a reddish eruption of a maculopapular character, occurring typically on the chest and back, was seen in about two-thirds of the cases. It differed from acne in the absence of pustules, and from sudamina in the absence of vesicles. It persisted into convalescence and was followed by scaling. Profuse sweats occurred in the highly toxic and in convalescents.

The case fatality of the pneumonias of this outbreak varied from 19 per cent in some of the southern camps to 51 per cent at Camp Sherman, Ohio. Certain camps in the same State and having apparently the same class of troops showed variation in this respect, thus suggesting that different standards were adopted in the diagnosis of pneumonia. From the account of symptoms and physical signs given above it is easily seen how this could have happened. During the fall wave the greatest percentage of total strength dying as a result of the epidemic was 3.3 (Camp Sherman, Ohio, and Camp Cody, N. Mex.). From this figure it ranged down to less than 0.5 per cent.

COMPLICATIONS AND SEQUEL?

Aside from the pneumonias which have been considered above, there was a notable absence of complications of influenza. Of 734,397 cases admitted with a primary diagnosis of influenza from our troops in the United States and in Europe, the following secondary diagnoses were recorded.¹

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Epidemic cerebrospinal meningitis	542	Phlebitis	225
Acute articular rheumatism	396	Bronchitis	5,081
Pulmonary tuberculosis	956	Bronchopneumonia	52,463
Acute miliary tuberculosis	21	Lobar pneumonia	21,742
Arthritis	916	Empyema	2,129
Hyperthyroidism	143	Serofibrinous pleuritis	904
Neurocirculatory asthenia	465	Pulmonary emphysema	61
Neuralgia	89	Asthma	229
Neuritis	143	Ulcer of the stomach	22
Psychasthenia	14	Ulcer of the duodenum	15
Psychoneurosis	114	Diarrhea	366
Psychosis, manic-depressive	25	Enterocolitis	553
Conjunctivitis	299	Hernia	516
Amaurosis	2	Anal fistula	30
Iritis	31	Cholecystitis	80
Otitis media	3,431	Peritonitis	46
Laryngitis (acute catarrhal)	490	Acute nephritis	313
Acute tonsillitis	2,617	Pyelitis	41
Acute pharyngitis	678	Cystitis	69
Mastoiditis	423	Epididymitis (nonvenereal)	163
Otitis externa	48	Abscess, subcutaneous	370
Sinusitis (all)	1,023	Furuncle	205
Pericarditis	139	Cellulitis	164
Acute endocarditis	50	Erythema	44
Cardiac dilatation	109	Herpes	60
Myocarditis and myocardial insufficiency	330	Herpes	60

Though it is undoubtedly true that, in the press of work occasioned by the epidemic, many minor complications were not recorded, the above figures probably represent with some accuracy the incidence of complications and sequel? important enough to have an effect on the clinical course of the case. It is seen that, with the exception of the respiratory complications, the number recorded under any heading is very small in proportion to the total number of admissions.

A number of reports on the incidence of otolaryngological complications have been published. Reports of the symptomatology of the disease indicate that catarrhal otitis media without perforation was of frequent occurrence, though exact figures are not available. The figures in the above tabulation probably represent fairly accurately those cases of otitis that required special attention or operation. It is seen in the tabulation that this complication occurred in 3,431 cases out of 734,397, or only 4.68 instances per 1,000 influenza admissions. The rate of secondary otitis media for all medical cases, exclusive of influenza, is 5.01 per 1,000. Measles had a rate of 41.9 per 1,000, scarlet fever 35.2, and epidemic meningitis 21.6. There is no doubt that there is some slight tendency toward lowered resistance in parts other than the lungs during the course of influenza, but were the statistics available it is certain that it would appear that the vast majority of these infections were associated with influenzal pneumonias of corresponding bacterial origin rather than with the primary disease itself.

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In general, complications were less frequent proportionately during the height of the fall outbreak than was the case during the earlier waves and during the period of decline after the fall wave. This is particularly true of the incidence of empyema.^a Several complications, while not numerically important, are of great interest on account of the fact that they appear to have attracted particular attention for the first time during this pandemic.

The first of these is subcutaneous emphysema.⁸⁷In this condition the subcutaneous areolar tissue becomes infiltrated with air over a greater or lesser extent of the body. It usually began above the clavicles or manubrium and extended in some instances until practically the entire body was affected. Its occurrence was extremely irregular, some large series of cases being reported without noting it and others reporting several cases in a comparatively small number of admissions. Camp Hospital No. 12, A. E. F., reported 13 cases, of which 5 occurred in the same ward and the first 3 in adjoining beds. This distribution led to the consideration of an infective origin for the complication and from 4 of the cases an anaerobic spore-bearing gas former was isolated. However, the great majority of cases failed in other hands to show any such origin, and the generally accepted theory of the pathogenesis of the emphysema, based on careful autopsy studies, is that it is the result of rupture of the dilated bronchioles, the air passing along the vessel sheaths to the mediastinum and thence to the subcutaneous tissue. The slow dissection thus accomplished by the air is remarkably painless; and while its occurrence is prognostically bad, by no means all of the extensive cases were fatal.

Another interesting complication is the degeneration of the rectus muscle, usually accompanied by rupture and hemorrhage. After attention was called to this occurrence a few instances were reported in almost every autopsy series. McCallum⁷⁹noted it in the 1918 spring epidemic. The primary lesion appears to be a hyaline degeneration of the muscle fibers with loss of striation, similar to if not identical with the condition known as Zenker's degeneration. When rupture and hemorrhage are added, bacterial invasion of the area may result in the formation of abscess. This condition doubtless accounts for many of the instances of abdominal pain and rigidity, simulating peritonitis, that were observed during the epidemic.

Inflammation of the accessory sinuses of the nose, while rarely giving rise to clinical symptoms, was almost invariably found post-mortem. The postorbital headache of the early days of the disease has been attributed to sphenoidal sinusitis. There has been a fairly prevalent belief that influenza was frequently followed by pulmonary tuberculosis. It is seen from the tabulation given above that in 956 instances the diagnosis of tuberculosis, secondary to influenza, was recorded. This amounts to 1.3 instances per 1,000 admissions. In all noninfluenzal admissions this diagnosis was recorded secondarily in 1.5 of every 1,000. These figures should perhaps be accepted with caution owing to the fact that the great majority of the men affected by influenza were discharged from the service within a few months time and late-developing tuberculosis might have been missed. However, the mortality statistics of the registration area for the years following 1918 have shown a progressive decrease in the death

^aEmpyema is given separate consideration in Pt. II, Vol. XI, of this history.

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rate from tuberculosis in civil life. It seems very improbable that any great number of cases of tuberculosis owe their origin to the influenza epidemic. The same general conditions hold for neurocirculatory asthenia. Here, again, although 465 cases are recorded as following influenza, the rate per 1,000 is lower than that for cases that were not influenzal.

TREATMENT

In the absence of definite knowledge of the etiology of influenza, no specific remedies are available for its cure. The current conception of this disease as a relatively mild respiratory infection, short in duration and leading to fatal results only when complicated by secondary infections, usually pulmonary, results in a treatment logically directed to shorten its course, to limit the amount or degree of primary pulmonary damage, to protect the patient against secondary infection from his fellows, and to reduce if possible by these means the proportion of fatal pulmonary complications. Further treatment aiming to promote the comfort of the patient is the second line of attack. When pneumonia has developed there are several methods of treatment in influenzal cases that do not apply in interepidemic periods, but in general the disease is best treated along orthodox lines.

INFLUENZA

Since the main aim of treatment in the uncomplicated early influenza case is the avoidance of pulmonary complications, the results of treatment are best estimated by consideration of the percentage of recoveries without pneumonia. The principles of treatment best adapted to this end have been well established, although definite statistical evidence of the same can not be given here owing to the fact that other factors predominated in determining the severity of cases as occurring in different localities. These factors have been included in the consideration of the epidemiology and of the prevention of the disease (vide supra).

Experience, however, led to the crystallization of the general opinion that certain measures resulted in reducing the proportion of pneumonia cases. Of these the first and perhaps most important was the early institution of treatment. Men who continued on duty after definite symptoms had developed were much more likely to develop pneumonia. The excellent morale of the combatant troops in the face of the enemy, which led many soldiers to refuse to report themselves sick until forced to do so, is believed to be one great cause of the greater proportion of pneumonias and relatively high fatality shown by the troops in the American Expeditionary Forces.⁸⁸The important elements of treatment, once the patient comes under medical care, were found to be rest in bed, warmth, and a light, hot diet. It is the consensus of opinion that under such treatment the great majority of cases are convalescent within two to three days. The question of open-air treatment has been much debated, but the weight of opinion is to the effect that open-air treatment is only permissible when it may be maintained without sacrificing the warmth of the patient. Drug treatment is of a palliative character. Aspirin was largely used for the pains of onset, though it was criticized by some as being depressant. Dover's powder, or morphine, to promote rest; sprays, preferably oily, to relieve naso-

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pharyngeal discomfort, and laxatives as needed comprise most of the drugs used. One report is available of the use of serum from convalescents in early cases. This showed that of 26 cases so treated only one-third the proportion of pneumonias resulted as in the untreated series and the average duration of fever was over 50 per cent longer in the controls.⁸³

PNEUMONIA

The general principles applicable to the uncomplicated influenza cases in regard to rest, warmth, and ventilation apply equally here. The usual drug medication was generally used without striking success. Specific treatment with antipneumococcus serum in type I cases was generally used and showed generally satisfactory results.⁸⁹Other attempts at specific treatment directed against the pneumococcus included the use of polyvalent antipneumococcus sera, the use of the Kye's antipneumococcus chicken serum, the autolyzed pneumococcus antigen of Rosenow, and the therapeutic use of bacterial vaccines.⁹⁰ Favorable reports on all these measures have been made by those who used them, as was also the case with the use of the serum or citrated blood of convalescents. It is to be noted that all except the last of these measures involves the introduction into the circulation, usually intravenously, of protein products foreign to the human system. This is also true of the type I serum, the effects of which, however, are so much more clean-cut than those of the others that its specific action may hardly be questioned. These considerations have led many to the belief that a nonspecific protein reaction is of benefit and some have aimed in their treatment to obtain a sharp reaction. To quote a report from Camp Greene, N. C.:⁹¹

It was the impression of some observers that not a few cases reported as other than type I showed benefit from the serum treatment. It was also the prevalent belief that in cases in which a chill follows the administration there was increased likelihood that 12 hours later the temperature would be much lower and the general condition improved.

Reports of the intravenous use of bacterial vaccines in doses sufficient to induce sharp reactions, repeated daily, show definitely good results.⁹²It appears to be quite definitely proven that such induced reactions do good. No harmful effects have been reported, and the claim is that the temporary discomfort of the chill is followed by a feeling of comfort and well being very grateful to the patient.^70,93

The intravenous use of hypertonic glucose solutions, while lacking the protein element, also results in a similar type of reaction in a certain proportion of cases. The advocates of this method of treatment, after extensive trials, claim that its use promotes comfort, produces rest and sleep, reduces temperature, increases elimination through kidneys and skin, slows the heart, and increases the volume of the pulse. It also supplies nutriment in a readily assimilable form and furnishes an excellent vehicle for the administration of specific sera, digitalis, morphine, or other medication. Comparisons showing reduction of mortality under strictly controlled conditions as a result of this treatment are not to be had. The method was used most extensively at Camp Travis and Fort Sam Houston, Tex., situated in the region where relatively low fatality prevailed. It has already been noted, however, that the case

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fatality there was appreciably lower than in neighboring camps under apparently the same climatic conditions and made up of the same type of men. The solution, from 5 to 25 per cent in strength, was given in amounts of 250 c. c. intravenously once or twice daily.

Notes were made of marked improvement following several cases of lung puncture for diagnostic purposes. The induction of artificial pneumothorax resulted in recovery in two cases apparently moribund.⁸²The suggestion was made on the ground of autopsy findings that aspiration of the chest should be performed early when signs of pleural fluid were noticed.⁹⁴Venesection was used to some extent in the severe cases of hemorrhagic edema of the early stages of the 1918 fall outbreak. The excellent results obtained by this means in gas pneumonias, together with the similarity of the pathology in the two conditions, lead to the expectation of marked benefit.⁹⁵There is decided difference of opinion as a result of experience. Some have reported marked benefit, while others state that no results were obtained.

In general it may be said that the experience of the World War has confirmed the position of the antipneumococcus serum, type I, when given in sufficiently high titre and proper dosage; it has led to a widespread belief in the beneficial effect of nonspecific protein reactions however induced and an equal belief in their essential harmlessness; it has shown the beneficial effect of the intravenous use of hypertonic glucose solutions, although experience with this agent was not general; and, lastly, owing to the universal agreement of the many who made use of convalescent serum in some form as to the good effect of this treatment, it appears established that the serum of convalescents contains curative antibodies.⁹⁶This last observation encourages the hope that with the discovery or recognition of the primary etiological agent of influenza a hyperimmune serum may be developed in animals which may at least prove effective in limiting the amount of pulmonary damage done by the primary disease and thus prevent the pneumonic complications.

REFERENCES

(1) Based on sick and wounded reports made to the Surgeon General.

(2) Cole, R., and MacCallum, W. G.: Pneumonia at a Base Hospital. Transactions of the Association of American Physicians. Philadelphia, 1918, xxx iii, 229.

(3) Report of the pneumonia commission at Camp Wheeler, Ga, made to the Surgeon General, October, 1918. On file, Record Room, S. G. O., 334.8-1 (Camp Wheeler) D.

(4) Osler, Sir William, and McRae, Thomas: Modern Medicine. Lea and Febiger, Philadelphia and New York, 1913, Vol. I, 534.

(5) Based on Mortality Statistics, prepared by the Department of Commerce, Bureau of the Census.

(6) Based on Mortality Statistics, prepared by the Department of Commerce, Bureau of the Census; also Thirteenth Census of the United States, taken in the year 1910 Vol. I, Population 1910, Bureau of the Census.

(7) Based on Annual Reports of the Surgeon General, U. S. Army, 1865-1917.

(8) Based on sick and wounded reports made to the Surgeon General; also on strength reports made to The Adjutant General.

(9) Brownlee, J.: The Next Epidemic of Influenza. Lancet, London, Nov. 8, 1919, ii, 856.

(10) Hall, Milton W., Maj., M. C.: A Possible Fallacy in the Calculation of Annual Death Rates. The Military Surgeon, 1923, lii, No. 2, 157.

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(11) Reports on influenza epidemic, 1918, made by the camp surgeon, to the Surgeon General. On file, Record Room, S. G. O., 710-1 (name of camp) D.

(12) Vaughan, V. C.: Epidemiology and Public Health, St. Louis, C. V. Mosby Company, 1922, I, 357.

(13) Opie, E. L.; Blake, F. G.; Small, J. C.; and Rivers, J. M.: Epidemic Respiratory Diseases. St. Louis, C. V. Mosby Co., 1921.

(14) Vaughan, V. C., op. cit., 358-9.

(15) Report of the influenza epidemic at Camp Grant, Ill., made by the camp surgeon to the Surgeon General. On file, Record Room, S. G. O., 710-1 (Camp Grant) D.

(16) Letter from Maj. E. L. Opie, M. C., to the Surgeon General, July 29, 1918. Subject: Pneumonia in Camp Cody among newly drafted men. On file, Record Room, S. G. O., 710-1 (Camp Cody) D.

(17) Sanitary report, Camp Lewis, Wash., for September, 1918. Copy on file, Record Room, S. G. O., 721-1 (Camp Lewis) D.

(18) Vaughan, V. C., Col. M. C., and Palmer, G. T., Capt, S. C.: Communicable Diseases in the National Guard and National Army of the United States During the Six Months from September 29, 1917, to March 29, 1918. The Journal of Laboratory and Clinical Medicine, St. Louis, 1918, iii, No. 2, 635.

(19) Pearl, Raymond, Public Health Reports, August 8, 1919.

(20) Annual Report of the Surgeon General, U. S. Army, 1918, 178.

(21) Census Reports, 1918.

(22) Analysis of the course of epidemics in Army camps, made by Col. V. C. Vaughan, M. C., undated. On file, Record Room, S. G. O., 701 (Influenza).

(23) Vaughan, V. C., op. cit. (Ref. 12), 401.

(24) Brewer, I. W.: Report of epidemic of Spanish Influenza, which occurred at Camp A. A. Humphries, Va., during September and October, 1918. The Journal of Laboratory and Clinical Medicine, St. Louis, 1918, iv, 87.

(25) Lynch, C. and Cumming, I. G.: The R?le of the Hand in the Distribution of Influenza Virus, and the Secondary Invaders. The Military Surgeon, 1918, xlii, 597; also, The Journal of Laboratory and Clinical Medicine, St. Louis, 1918, v, 364.

(26) Blanton, W. R., and Irons, E. E.: A Recent Epidemic of Acute Respiratory Infection at Camp Custer, Michigan. The Journal of the American Medical Association, 1918, lxxi, No. 24, 1918.

(27) Vaughan, W. T.: Monograph No. 1. The American Journal of Hygiene, 1921.

(28) Jackson, Thomas W.: The Other Side of the Question of Indirect Contact Infection in Acute Respiratory Diseases. The Military Surgeon, Washington, 1920, xlvi, No. 5, 570.

(29) Hall, Milton W.: A Note on the Epidemiology of Influenza. The Military Surgeon, Washington, 1920, xlvi, No. 5, 564.

(30) Nichols, H. J.: Bacteriologic Data on the Epidemiology of Respiratory Diseases in the Army. The Journal of Laboratory and Clinical Medicine, St. Louis, 1920, v. 502.

(31) Rosenau, M. J.; Keegan, W. J.; Goldberger, J.; and Lake, G. C.: Some Interesting Though Unsuccessful Attempts to Transmit Influenza Experimentally. Public Health Reports, 1919, xxxiv, No. 2, 33. Also, McCoy, G. W., and Richey, De Wayne: San Francisco Experiments. Public Health Reports, 1919, xxxiv, No. 2, 34.

(32) Vaughan, V. C., op. cit. (Ref. 12), 400.

(33) Minaker, A. J., and Irvine, R. S.: Prophylactic Use of Mixed Vaccines against Pandemic Influenza and its Complications. The Journal of the American Medical Association, Chicago, 1919, lxxii, No. 12, 847.

(34) Vaughan, V. C., op. cit. (Ref. 12), 402-3.

(35) Report on influenza made by the surgeon, Camp Lee, Va. On file, Record Room, S. G. O., 710-1 (Camp Lee) D.

(36) Weaver, G. H.: The Value of the Face Mask and Other Measures in Prevention of Diphtheria, Meningitis, Pneumonias, etc. The Journal of the American Medical Association, Chicago, 1918, lxx, No. 2, 76.

(37) Capps, J. A.: A New Adaptation of the Face Mask in Control of Contagious Disease. The Journal of the American Medical Association, Chicago, 1918, lxx, No. 13, 910.

167

(38) Doust B. C., and Lyon, A. B.: Face Masks in Infections of the Respiratory Tract. The Journal of the American Medical Association, Chicago, 1918, lxxi, No. 15, 1216.

(39) Protection afforded by various filters against bacterial suspension in air-Tolman, Guernsey, Charleston and Dougherty. On file, Record Room, S. G. O., 729.2-1.

(40) Draper, G.: Some Observations on the Susceptibility of the Recruits to Disease. The Military Surgeon, 1919, xlv, 99.

(41) A summary of the epidemiological evidence as to the relation between typhoid inoculation and the incidence of pneumonia, made by Maj. A. W. Freeman, M. C. On file, Record Room, S. G. O., 334.8-1 (Camp Funston) D.

(42) Medlar, E. M.: Effect of Typhoid Lipovaccine in increasing Susceptibility to Other Diseases. The Journal of the American Medical Association, Chicago, 1918, lxxi, No. 26, 2146.

(43) The relation of typhoid vaccination to the incidence of pneumonia. On file, Record Room, S. G. O., 334.8-1. (Camp Wheeler) D.

(44) Based on monthly sanitary reports, made by camp surgeon. On file, A. G. O., World War Division.

(45) Stanley: Public Health Reports, May 9, 1919.

(46) Miller, J. L., and Lusk, F. B.: Epidemic of Streptococcus Pneumonia and Empyema at Camp Dodge, Iowa. The Journal of the American Medical Association, Chicago, 1918, lxxi, No. 9, 702.

(47) The streptococcus epidemic at Camp Zachary Taylor, Ky., by Majs. H. Fox, M. C., and W. W. Hamburger, M. C. On file, Record Room, S. G. O., 710-1 (Camp Zachary Taylor) D.

(48) Cecil, E. L., and Austen, J. H.: Results of Prophylactic Inoculations against Pneumococcus in 12,519 men. The Journal of Experimental Medicine, 1918, xxviii, 19.

(49) Cecil, R. L., and Vaughan, H. F.: Results of Prophylactic Vaccination against Pneumonia at Camp Wheeler. The Journal of Experimental Medicine, 1919, xxix, 457.

(50) Report of vaccination of personnel of Camp Lusitania, A. E. F., with pneumococcus and B. influenza vaccine, July 2, 1919. On file, Record Room, S. G. O., 720.3-1 (A. E. F., France) Y.

(51) Reynolds, J. R.: A System of Medicine. J. B. Lippincott & Co., Philadelphia, 1870, Vol. I, 36.

(52) Annual Report of the Surgeon General, U. S. Army, 1919, Vol. I, 504.

(53) Vaughan, V. C., op. cit. (Ref.12), 367.

(54) Bradbury, S.: The Occurrence of Influenza in a Regiment. The Military Surgeon, Washington, 1920, xlvii, No. 4, 471.

(55) Report from Camp Dodge. On file, Record Room, S. G. O., 710-1 (Camp Dodge) D.

(56) West, S.: Some General Remarks on Epidemic Influenza. The Practitioner, London, January-June, 1919, cii, 44.

(57) Annual Report of the Surgeon General, U. S. Army, 1919, Vol. I, 746.

(58) Chesney, Alan M., and Snow, F. W.: A Report of an Epidemic of Influenza in an Army Post of the American Expeditionary Forces in France. On file, Record Room, S. G. O., 710-1 (A. E. F.; France) Y.

(59) Special report from Camp Shelby, Miss. On file, Record Room, S. G. O., 700-1 (Camp Shelby) D.

(60) Report from the base hospital, Camp Devens, Mass., relative to influenza, pneumonia, and respiratory diseases. On file, Record Room, S. G. O., 710 D.

(61) Special report from Camp Logan, Tex. On file, Record Room, S. G. O., 710-1 (Camp Logan) D.

(62) Special report from Camp Cody, N. Mex. On file, Record Room, S. G. O., 710-1 (Camp Cody) D.

(63) Report from the base laboratory, Base Section No. 5, Brest, A. E. F. On file, Record Room, S. G. O., 710-1 (A. E. F., France) Y.

(64) Brown, O. G.: Problems in the Control of Communicable Diseases at Replacement. Depots. The Military Surgeon, Washington, 1919, xlv, No. 1, 59.

168

(65) Cornwall, E. E.: Spanish Influenza; Cases of Influenza and Pneumonia taken off S. S. Bergernsfjord, arrived in New York, August 12, 1918, from Norway. New York Medical Journal, August 24, 1918, cviii, 330.

(66) Freedlander, A.; McCord, C. P.; Sladen, F. J.; and Wheeler, G. W.: The Epidemic of Influenza at Camp Sherman, Ohio. The Journal of the American Medical Association, Chicago, 1918, lxxi, No. 20, 1652.

(67) Lyon, J. P.; Tenney, C. F.; and Szerlip, L.: Some clinical observations on the Influenza Epidemic at Camp Upton. The Journal of the American Medical Association, Chicago, 1919, lxxii, No. 24, 1726.

(68) Report from the camp surgeon, Camp Grant, Ill., relative to influenza epidemic. On file, Record Room, S. G. O., 710-1 (Camp Grant) D.

(69) Report from the camp epidemiologist, Camp Devens, Mass., relative to influenza epidemic. On file, Record Room, S. G. O., 710-1 (Camp Devens) D.

(70) Report from Camp Travis, Tex., relative to influenza epidemic. On file, Record Room, S. G. O., 710-1 (Camp Travis) D.

(71) Based on reports from base hospital laboratories, relative to typing the pneumococcus. On file, Record Room, S. G. O., 710 (Pneumonia).

(72) Chickering, H. T., and Park, J. H.: Staphylococcus Aurens Pneumonia. The Journal of the American Medical Association, Chicago, 1919, lxxii, No. 9, 617.

(73) Nichols, H. J., and Stimmel, C. O.: Pneumobacilli as Complicating Organisms in Influenzal Pneumonia. The Journal of the American Medical Association, Chicago, 1919, lxxii, No. 3, 174.

(74) MacCallum, W. G.: Pathology of the Epidemic Streptococcal Broncho-pneumonia in Army Camps. Monograph of the Rockefeller Institute for Medical Research, 1919, No. 10; also The Journal of the American Medical Association, Chicago, 1918, lxxi, No. 9, 704.

(75) Opie, E. L., Freeman, A. W., Blake, F. G., Small, J. C., and Rivers, T. M.: Pneumonia Following Influenza at Camp Pike. The Journal of the American Medical Association, Chicago, lxxii, No. 8, 556.

(76) Winternitz, M. C., Wason, I. M., and McNamara, F. P.: The Pathology of Influenza, The Yale University Press, New Haven, Conn., 1920.

(77) Opie, E. L., Freeman, A. W., Blake, F. G., Small, J. C., and Rivers, T. M.: Pneumonia at Camp Funston. The Journal of the American Medical Association, Chicago, 1919, lxxii, No. 2, 108.

(78) Report on pneumonia at Camp Dodge, Iowa, by Maj. D. F. Dick, M. C. On file, Record Room, S. G. O., 710-1 (Camp Dodge) D.

(79) McCallum, W. G.: Pathology of the Epidemic Streptococcal Bronchopneumonia, in Army Camps. The Journal of the American Medical Association, Chicago, 1918, lxxi, No. 9, 704.

(80) Le Count, E. R.: Pathologic Anatomy of Influenzal Broncho-pneumonia. The Journal of the American Medical Association, Chicago, 1919, lxxii, No. 9, 650.

(81) Wolbach, S. B.: Comments on the Pathology and Bacteriology of Fatal Influenza Cases As Observed at Camp Devens, Mass. Johns Hopkins Bulletin, Baltimore, 1919, xxx, No. 338, 104.

(82) Horkavy, J., and Selby, John H.: Report on a Clinical, Pathological, and Several X-ray Studies of the Influenza Epidemic at Walter Reed General Hospital. On file, Record Room, S. G. O., 710-1 (Walter Reed G. H.) K.

(83) Francis, F. D., Hall, M. W., and Gaines, A. R.: Early Use of Convalescent Serum in Influenza. The Military Surgeon, Washington, 1920, xlvii, 177.

(84) Blanton, W. B., Burhans, C. W., and Hunter, O. W.: Studies in Streptococcal Infections at Camp Custer, Michigan. The Journal of the American Medical Association, Chicago, 1919. lxxii, No. 21, 1520.

(85) Forbes, Roy P., and Snyder, Helen A.: A Study of the Leucocytes in An Epidemic of Influenza at Camp Hancock, Ga. On file, Record Room, S. G. O., 710-1 (Camp Hancock) D.

(86) Report on a clinical study of influenza pneumonia by Lieut. A. D. Rood, M. C. On file, Record Room, S. G. O., 710-1 (Walter Reed G. H.) K.

169

(87) Subcutaneous emphysema complicating broncho-pneumonia, Lieut. D. M. Nyquist, M .C. On file, Record Room, S. G. O., 710-1 (A. E. F., France) Y; also, Influenza pneumonia cases showing gas in the fascial tissues, Majs. E. Clark and M. J. Synnott, M. C. On file, Record Room, S. G. O., 710-1 (Camp Dix) D; also, Generalized interstitial emphysema and spontaneous pneumothorax as complications of broncho-pneumonia, Capts. H. K. Berkley and T. H. Coffen. On file, Record Room, S. G. O., 710-1 (Camp Lewis) D.

(88) Brooks, H., and Gillette, C.: The Argonne Influenza Epidemic. New York Medical Journal, New York, 1919, cx, No. 23, 925.

(89) Spooner, L. H., Sellards, A. W., and Wyman, J. H.: Serum Treatment of Type I Pneumonia. The Journal of the American Medical Association, Chicago, 1918, lxxi, No. 16, 1310. Also, A group of 68 cases of Type I pneumonia occurring in 30 days at Camp Upton, with special reference to serum treatment. Maj. C. F. Tenney and Lieut. W. T. Riverburgh, M. C. On file, Record Room, S. G. O., 710-1 (Camp Upton) D.

(90) Antipneumococcus serum (Kyes') in the treatment of pneumonia, Maj. A. W. Gray, M. C. On file, Record Room, S. G. O., 710-1 (Camp Grant) D. Also: Antipneumococcus serum (Kyes') in the treatment of lobar pneumonia, John H. McClellan, M. D. On file, Record Room, S. G. O., 710-1 (Camp Grant) D.

(91) Brown, C. P.. and Palfrey, F. W.: Influenzal Pneumonia at Camp Greene, N. C. New York Medical Journal, August 23 and 30, 1919, cx, 316, 368.

(92) Roberts, D., and Cary, E. G.: Bacterial Protein Injections in Influenzal Pneumonia. The Journal of the American Medical Association, Chicago, 1919, lxxii, No. 13, 922.

(93) Snyder, R. G.: Spanish Influenza; Its Treatment by the Use of Intravenous Injections of a Non-Bacterial Split Protein. New York Medical Journal, New York, 1918, xviii, 843. Also, The use of intravenous injections of hypertonic glucose solutions in the treatment of influenzal pneumonia. Capt. C. W. Wells and Lieut. R. C. Blankenship. On file, Record Room, S. G. O., 710-1 (Camp Travis) D. Also: Glucose as an adjunct measure in the therapy of pneumonia, Capt. Henry J. John, M. C. On file, Record Room, S. G. O., 710-1 (Ft. Sam Houston) N.

(94) Report of the pathology of influenza and pneumonia, Capt. James F. Coupal. On file, Record Room, S. G. O., 710-1 (A. E. F., France) Y.

(95) O'Malley, J. J., and Hartman, F. W.: Treatment of Influenzal Pneumonia with Plasma of Convalescent Patients. The Journal of the American Medical Association, Chicago, 1919, lxxii, No. 1, 34.

(96) McQuire, L. W., and Redden, W. R.: Treatment of Influenzal Pneumonia by the Use of Convalescent Human Serum. The Journal of the American Medical Association, Chicago, 1918, lxxi, No. 24, 1992. Also: Ross, C. W., and Hund, E. J.: Transfusion in the Desperate Pneumonias Complicating Influenza. Journal of the American Medical Association, Chicago, 1918, lxxi, No. 24, 1992. Also: Kahn, M. H.: Serum Treatment of Postinfluenzal Broncho-pneumonia. Journal of the American Medical Association, Chicago, 1919, lxxii, No. 2, 102. Also: Maclachlan, W. W. G., and Fetter, W. J.: Citrated Blood in the Treatment of the Pneumonia Following Influenza. Journal of the American Medical Association, Chicago, 1918, lxxi, No. 25, 2053. Also: Browne, W. L., and Sweet, B. L.: Treatment of Influenzal Pneumonias by Citrated Transfusions. Journal of the American Medical Association, 1918, lxxi, No. 19, 1602.