History

Medical Department of the U.S. Army in the World War

CHAPTER II

INFLAMMATORY DISEASES OF THE RESPIRATORY TRACT(BRONCHITIS, INFLUENZA, BRONCHOPNEUMONIA,

(LOBAR PNEUMONIA)

In approaching the consideration of the serious and fatalinflammations of the respiratory tract which formed by far the most importantfactor in the sickness and death records of the Army during the World War, it isfirst necessary to take a general view of the subject in the attempt todetermine, if possible, the causes that led to the large morbidity and mortalityfrom respiratory diseases in general, rather than to limit ourselves to theconsideration of each form of disease separately.

The mortality from respiratory diseases during the World Warwas due almost entirely to pneumonia, primary or secondary.¹ In any set ofcommunities the size of the mobilization camps of the Army during the war,pneumonia is to be expected to some extent. The usual type of pneumoniaoccurring among young male adults in civil life is of course primary lobarpneumonia, running a fairly definite course and, usually, recognized easily bothclinically and post mortem. That such cases occurred among the troops is beyondquestion. 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 numberand distinguished mainly by their mildness as compared to those seen in civilcommunities.²However, it was early recognized clinically that in thelarger number of cases observed in the camps the pneumonia was of an atypicalnature. The onset tended to be slower than that of the lobar pneumonia of civillife; the course more prolonged. Crisis was relatively rare; physical signs wereslow of development and of patchy distribution and scattered in several lobes.These facts led careful observers to consider a large proportion of the cases asbronchopneumonia rather than as the usual lobar type. The results of post-mortemstudy of fatal cases lent confirmation to this distinction: The typical croupousconsolidation of lobar pneumonia was relatively rare, patchy consolidation of asuppurative character more frequent. Even when the consolidation involved nearlyor quite an entire lobe, careful study often showed evidence of the formation ofsuch lobar consolidation by the confluence of smaller areas, lobular in origin.

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

^aUnless otherwise stated, all figures for the World War periodare 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-calledendemic form, was a considerable factor in the sick rates of the Army for someyears before the World War. It has always been held responsible for a small butvarying mortality in the civil population according to the reports of the CensusBureau. It is the consensus of opinion of those who have investigated thesubject that minor but distinct epidemic waves of this disease have occurredevery few years, in each instance accompanied by an increase in the pneumoniamortality. Cases of influenza were reported from the camps from the earliestdays of the mobilization; doubtless many more cases were not recognized owing tothe mildness of the type prevailing during the fall of 1917. It is impossible,therefore, to estimate the number of influenza cases that occurred among thetroops during these early months. But that the disease was present will be shownin a manner that will leave very little room for doubt, and its fluctuationsfrom month to month, as shown by its effect on the number of admissions for thetotal respiratory diseases and by its effect on the amount and character of theprevalent pneumonia, can be shown with some definiteness.

Unfortunately for the exactness of our records in this classof diseases the clinical characteristics of mild influenza are such as to leadto its ready confusion with several of the milder so-called common respiratorydiseases. Of these, bronchitis, tonsillitis, and pharyngitis are the leadingdiseases with which many of the earlier cases of influenza were confused. Whenthe outbreak was at its height the uniformity of symptoms presented by largenumbers of cases made confusion almost impossible and at the time of an epidemicwave in the majority of instances the cases were correctly diagnosed. However,in certain camps there were pathological purists who refused to give sanction tothe diagnosis of influenza unless it was possible to demonstrate the presence ofthe bacillus of Pfeiffer. This attitude was evidently extreme, in view of thedoubt cast in recent years on the specificity of the role of this organism ininfluenza; but the fact remains that in all of the epidemic waves to bedescribed, even in the generally recognized fall outbreak of 1918, there was notonly an increase in the number of cases diagnosed as influenza but also acorresponding increase in the "other respiratory diseases." One campreported a preponderant number of influenza cases, another simultaneouslysuffering from the same epidemic wave reported few influenza cases, but a greatincrease in the common respiratory diseases. Even in the 1918 fall wave, threecamps-Fremont, Calif.; Gordon, Ga.; and Wheeler, Ga.³-apparentlyinsisted on a bacteriological diagnosis, which was not forthcoming, and reportedtheir epidemic cases as "other respiratory diseases." These twofactors then, the impossibility of making an exact clinical diagnosis ofinfluenza in the absence of the great outbreak, and the insistence by some onthe bacteriological diagnosis even in the presence of undoubted waves of thedisease, make it impracticable to base conclusions as to the varying incidenceof influenza in the Army camps on the reported cases of that disease alone. Instudying 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		Broncho pneumonia		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		Broncho pneumonia		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 consistentlyhigher 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 thanthe total white rate (9.75). It about equals the rate observed for the whitetroops in Europe (7.75). In view of their high admission rate, their lowfatality 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 ofthese men were stationed. This applies also to the low rates for the PhilippineScouts (3.72) and the Hawaiians (2.31). The death rates for officers (4.87) werelower than for enlisted men (11.09). This difference is more marked in theUnited States than in Europe. The influence of environment is markedly shown in

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

CHART VII.-The relations between the annual admissionratesper 1,000 strength, white enlisted men in the United States, of the combinedinfluenza 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 influenzaepidemic, use has been made of the admission and deathrates of influenza, bronchitis, bronchopneumonia, and lobar pneumonia, combinedin each case into a single rate. From the number of admissions and the number ofdeaths, the case fatality has been calculated. Tables 13 to 22, inclusive, andCharts 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 thecharts referred to in one or more of three ways, increases being in theadmission rate, in the death rate, or in the casefatality. Differences in race or in length of service areaccompanied by differences in the wayin which a group reacts to the presence of influenza. Whenthe epidemic is at its worst all three factors are

markedly affected, but in the case of milder waves only that factor to whichthe group responds most sensitively may show indications of the presence of theepidemic wave. Thus, in general, the colored race shows less variation in case fatality and more in morbidity than does the white race. So, too, theprocess of "seasoning" alters the relation of the case fatality andthe admission rates to the death rates. With these preliminary facts in mind itis 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 somedegree of probability those months in which an epidemic wave had its occurrence. While statistics classified by weeks would be much more desirable forthis purpose, such are not available, and the figures by months give a veryclear-cut picture.

CHART IX.-Annual admission and death rates per1,000strength for white and colored enlisted men in Europe, total respiratory groupof diseases, by months, June, 1917, to December, 1919. (No rates shown whenstrength was
less than 1,000)

CHART X.-Annual admission and death rates per1,000 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 1,000strengthfor 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 ofdiseases 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 than1,000.

TABLE 15.-Annualadmission 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 per1,000 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 1,000strength, 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, coloredenlisted 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, whiteenlisted 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	Broncho pneumonia	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 bothhigh for white enlisted men in the United States in April, 1917; not as high asin the subsequent April, but in relation to the low rates of the followingsummer, the difference is almost as marked. Was this high point due to influenzaat that early date, or was it merely an unusual seasonal prevalence ofrespiratory 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 theArmy was comparatively small; in the main it consisted of seasoned troops.Consequently any considerable rise in rates at this time is of more significancethan would be the case had a large proportion of recruits recently joined. Thecase fatality

rate for white enlisted men at this time was exceeded onlytwice in the subsequent months, during the great wave of the fall, and in thefirst recurrence thereafter. It seems justified to conclude that we have herefairly definite evidence to the effect that the influenzal infection was then atwork among the troops. The first recognizable wave then during the war periodoccurred in the first month shown in the statistics. Following this wave therewas 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 inJanuary 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 therates for all groups, to be followed by a third and, in this country, muchhigher 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 asin the United States. The admission rates rise slightly in March and then fallvery slowly for the succeeding months, apparently continuing high through June.This was the period of the type of influenza known to the AmericanExpeditionary Forces as "three day fever." Except in the first monthit was accompanied by relatively little mortality. The next rise shown is thatculminating in the great October peak. There is in all groups studied a higheradmission and death rate for respiratory diseases in August than there was forJuly. This is a significant phenomenon taken in connection with the fact thatAugust is usually expected to show the lowest rates for these diseases of theentire year. In the colored troops in the United States this rise is seen tobegin a month earlier still, or in July, 1918. At this time, a large number ofcases of rather fatal lobar pneumonia were noted in this race in widelyscattered camps. It seems probable from the progressive character of thesubsequent rise in the rates that these cases represent the earliest of thegreat wave, occurring in the most susceptible group in the Army. The casefatality for white troops also showed a rise in the month of July and progressively continued into the high point of thefall wave. To determine the actual time of beginning of the rise for this waverecourse is had to a compilation of the weekly telegraphic reports of currentmedical statistics. Though these are not satisfactory for the purpose ofestimating the total number of cases, they do show well the comparative numbersfrom week to week. Influenza was not on the list of diseases of which weeklyreports were required at that time, and so the compilation includes only thosestations that did report the disease. The data are assembled in Table 23. Thereis seen to be a progressive increase in cases reported as influenza beginningwith the week ending August 4, 1918, and of the influenzal pneumonia casesbeginning with the week ending August 18 of that year. If this was really thebeginning of the great epidemic wave we should expect that if these series ofdata were plotted out on a logarithmic scale the increase from week to weekwould plot out as a straight line following the usual logarithmic rise of anepidemic curve. That this condition is substantially fulfilled is seen in ChartXIV.

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 countryand in Europe the rates for respiratory disease began to rise at least as earlyas the month of August and that the rise was practically simultaneous in the twoforces, separated by thousands ofmiles of water. That an epidemicwave once developed is spread by contact ofcases, is of course, incontrovertible. But that thewidespread, practically simultaneous, increase in therates that was observed not only with this wave but alsowith all the preliminary and recurrent waves of thepandemic could have been accounted for by transmissionfrom 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) Thebeginningof of the 1920 recurrence observed in December, 1919.

Of these waves, the first, second,fourth, sixth, and seventh (culminating as it did acouple of months later) fall atintervals that coincide very well with the 33-week period of influenza pointedout by Brownlee.⁹The third and fifth waves, whichdo not fit intothis scheme, also fall about that time apart. The ArmyMedical Department statistics of the World Warare not particularly

well adapted to the study of periodicity in its most exactform, since these statistics cover too large an assemblage of widely separatedunits; however, the distinct approximation to the period of Brownlee is ofmore than passing interest. The pointof greatest importance to be brought out at this time is the very evident factthat the time relations of the various waves described in

the troops at home and abroad seem topreclude the idea of the transmission of these waves from one command toanother. The peak months varied in some instances but the evidences of the beginningof the rise for each wave were usually coincident in the two groups. This observation is of fundamental importance in theepidemiology of the disease.

CHART XIV.-Incidence of influenza and influenzal pneumonia, by weeks, forcertain camps inthe United States, June 17to December 29, 1918. Annual admission rates per 1,000 strength based on weeklyreports 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 madeonly 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 thewhole Army, which included officers, and also enlisted men in various tropicalstations, both of which classes showed a decidedly lower death rate than did theenlisted men in this country and in Europe. Figures, by months, for the entireArmy, however, are not available, and inasmuch as the figures used include thegreat majority of the Army, the comparison seems a fair one.

The proportion of men of different ages in the Army duringthe war is not a matter of direct record; however, from the records of the WarRisk Insurance Bureau it is possible to show the ages of 3,673,125 men,obtained from their applications for policies of Government insurance. Since itis fair to assume that the average period of service was in the neighborhood ofone year, and that the insurance policy was taken out at or near the beginningof the service period, the ages of these men have been advanced six months eachto obtain an average figure for the war period. The following table shows theage 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 representativeof the actual conditions, although the fact that officers as well as enlistedmen 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 allkinds 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 totalnumber of deaths during the year be divided by the death rate and multiplied byone thousand it will give a figure representing the strength that would havegiven the same number of deaths during the year had the strength and the deathrate 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 thecorresponding months. For the reasons given above this is the best figure to usefor the average strength of the Army when comparing its rates with those of thecivil population.

The next factor to be estimated is the proportion of thevarious age groups of the male population as estimated for 1918 that were not inthe services (Army, Navy, and Marine Corps) and so remained to contribute to thecivil rate. The usual arithmetical method of estimating population based on thecensus returns of 1910 and of 1920 can not be applied to the years previous to1919 without taking into consideration the excess deaths in the various agegroups due to the influenza epidemic and to battle casualties. If these beestimated as carefully as possible and added to the population as found in 1920the estimate for 1918 becomes satisfactory. If, then, from the population ofeach age group, as thus estimated, is subtracted the number of men of each agein the service the remainder will represent the number in each age group thatcontributed to the civilian death rates. It is estimated that the Armyrepresented 87.5 of the total military forces during the war, the balance beingthe Navy and the Marine Corps. Hence the weighted mean strength of the servicesduring 1918 was 3,180,000 men. The registration States furnished 76.51 per centof this total or 2,435,000. If these men be divided into age groups accordingto the percentages shown in the table of ages the results will represent thenumber of men in each age group furnished to the services by the registrationStates. These numbers subtracted from the estimated population of theregistration area in each group will give the civil population in each groupduring 1918, and from these last figures the death rates for each group may becalculated from the number of deaths in each group in the registration states asfurnished by the Census Bureau. The following specific death rates from diseasefor 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 ofthe Army and the latter divided into age groups according to the age tablegiven, the total result is 42,184 deaths, or 15.1 per thousand on the weightedmean strength.

The comparative rate for the Army as given above was 16.1, adifference 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 diseasesare treated in the same manner the rates obtained are 12.59 for the Army and9.96 for the civil population. It is evident that the Army had a decidedlyhigher reported death rate for the respiratory diseases than was found in civillife and equally evident that the reverse was true for other causes of death. Infact, the figures indicate that at civil rates the Army would have lost nearlyseven thousand more men than was actually the case from causes other than therespiratory diseases.

The reasons for these differences are probably several. Itwill be shown later that men from rural districts, when introduced into theconditions of barrack life, are much more susceptible to respiratory infectionsthan are their urban brethren. Urban rates for respiratory diseases are alwaysmarkedly higher than those of rural districts. In the Army hundreds ofthousands of young men from rural districts were living in the Army camps inconditions of closer contact than in ordinary city life. The majority of thesemen in this country at the time of the great epidemic were comparatively new tothe service, 76 per cent had seen less than four months' service,^a and hadlittle or no time for "seasoning." Certain cities in the countryshowed rates during the epidemic higher than those of the Army. For the lastfour months of 1918, had the rates that obtained in Philadelphia been applied tothe Army strength there would have been 39,894 deaths from disease and 33,287from the respiratory diseases as compared with 36,858 and 33,136, respectively,which actually occurred in the Army. Again, there is seen the greatly largernumber of deaths from causes other than the respiratory diseases occurring inthe civil population even during the epidemic months. So great a disparitysuggests some differences in the standards of diagnosis in the two sets offigures. That many deaths occurring in the registration area were really due toinfluenza or pneumonia while otherwise reported, is suggested by the notable andunseasonable increases during the epidemic period in deaths from a number ofother causes. It is undoubtedly true also that during this period, the countryover, a great number of deaths occurred that were never reported. The conditionsin many places were such that the keeping of accurate records was animpossibility. In the Army, however, every man had to be accounted for and thedeath records are as nearly accurate as it is possible to make them. It is to berecalled, too, that the Army rates as used in this comparison, excluded certaingroups of the Army which showed a decidedly lower death rate than those givenherein, and that the total rate for the Army would have been somewhat lower hadthe complete monthly figures been available for comparison. The conclusion seemsjustified, then, that the disparity existing between the two rates was not morethan is accounted for by the assembling of large numbers of country boys incamps under urban conditions.

^aEstimate made in manner described under "The effect oflength 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 theavailable figures such facts as may show the effect of a number of varyingfactors on incidence and mortality of the respiratory group of diseases duringthe war. These varying factors have been considered in connection with differentgroups of the troops concerned, and the attempt finally is made to correlate theknowledge thus gained into a concrete conception of the pandemic as a whole;they include age, length of service in the Army, race, nativity, climate andweather, and housing.

AGE

As to the effect of age on the incidence of respiratorydisease, we are able topresent figures for deaths only, classified according to age.¹ These apply tothe whole Army, wherever located, and include the deaths of officers as well asof 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 asdetermined in the calculation of the relative death rates for the Army and forcivil life and to a total of 37,002 deaths, the total for these diseasesreported 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 agegroups as those published from civilian sources, though here the group 21-25has a relatively higher rate than is usually given.

LENGTH OF SERVICE

It has long been known that men new to the military serviceare more liable to contract disease, especially disease of the classes underconsideration, than are men long in the service. It is possible with the data athand to present certain facts showing the degree of this increasedsusceptibility of the recruit.

The report cards of 34,446 deaths from influenza pneumoniashow the length of service of the patient at the time of his admission tohospital.¹ Of

these deaths 9,847 or 28.56 per cent occurred in men of lessthan 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, or7.66 per cent in men of 6 to 8 months' service 1,663, or 4.83 per cent inmen of 8 to 10 months' service, 1,198 or 3.48 per cent in men of 10 to 12months' 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 impossibleaccurately to estimate the proportion of men of each service group in the Armyfor the entire period of the war, inasmuch as the proportion was constantlychanging with the passage of time; however, an attempt has been made to averagethe proportions found in different months. The resulting relative strengthsprobably approximate the distribution of men at the time of the greatestmortality in the fall of 1918. Based on these relative strengths, rates havebeen calculated showing the relative differences in death rates of the differentlength of service groups; these are given in Chart XVI and show the same generalrelations as Chart XV. The former gives a better relative idea of the importanceof short service as a cause of death. It is seen that 60.46 per cent of alldeaths occurred in soldiers of less than 4 months' service. It is to be notedalso that the second bimonthly period shows a larger proportion than does thefirst. This is an unexpected finding and may be due to other factors thanincreased incidence among the men concerned.

It is obvious that if there had happened to be an unusuallylarge proportion of men of the two to four months' group service in the Armyat the time of the fall wave (1918) of influenza, and a correspondingly smallnumber of men of less than two months' service, the number of deaths chargedto the latter group would be relatively small in the total and those of theformer would be too large. The same principles apply as to the estimation ofdeath rates in commands of varying strength. As a matter of fact the number oftwo to four months' men was considerably larger in September and October,1918, than was that of the less than two months' service men. This conclusionhas been arrived at by studying the relations of the total enlisted strength ofthe Army from month to month. Thus, if the total strength was greater by 200,000one month than in the month previous there must have been in that month 200,000men of less than one month service. This gives us means of checking up on thefigures already given. It is available only during the period of progressiveincrease in Army strength, and takes into account no losses from death ordischarge, but it is believed that it is roughly satisfactory as a check. If thenumber of men of less than two months' service in each month from October,1917, to October, 1918, inclusive, be compared with the total enlisted strengthin the United States the same month, the percentage of recruits of that lengthof service may be obtained for each month. If, then, for each month thecorresponding percentage of deaths among enlisted men in the United States becalculated and the results added the sum shows the number of deaths that shouldhave occurred among these men had their rate been the same as that of all theother men. This forms a certain percentage of the total deaths in the wholeArmy, 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 frominfluenzal 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-tenthstimes 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-eighthundredths times the former figure. It is impossible to carry calculation toolder service groups inasmuch as after four months' service considerablenumbers began to go abroad, and the calculation of strength becomes lessaccurate. However, it is seen that the figures conform to those of the formerseries in showing that the man of two to four months' service apparentlyshowed a higher mortality from respiratory disease thandid those of less than twomonths. As stated above, there were about 30 per centmore men of the longer service group in the serviceduring the fall outbreak of influenza than there were of the less than twomonths' service men. This undoubtedly is the factor increasing their rate inthe total.

The unfavorable effect on camp ratesof the presence of a number of recruits is mentionedin a large number of reports of the fall epidemic (1918).¹¹ Camp Sherman, Ohio, had the highest mortality rate of anylarge camp; 46 per cent of its strength were classedas recruits. The rate of Camp Cody, N. Mex., was almost as high, with 69 percent recruits; Camp Grant, Ill., 40 per cent; CampForrest, 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 percent recruits; Las Casas, Porto Rico, with 5 per cent; Camp Sheridan, Ala., with6 per cent; Camp Eustis, Va., with 7 per cent; and Camp McClellan, Ala., with 10per cent. If the larger camps be divided into fourgroups according to mortality, we find the group withthe highest mortality had 41 per cent recruits, the second 31 per cent, thethird 22 per cent, and thefourth 16 per cent. This relation is the most clean cut of any found among thefactors influencing the comparative rates of the camps.

Many specific instances of the high mortality of new men ascompared to those of longer service are on record. At Camp Grant, Ill., forinstance, the September inductants lost 4 per cent of their strength, while theloss for the balance of the camp was less than 2 per cent. Vaughan said:¹²"Ifrecruits had not been sent to Grant in September, the camp mortality rate fromthe epidemic would have been1.7 per cent; 16,000 recruits raised the rate to 2.6 per centan increase of 53 per cent."

The report of theinfluenza commission working at Camp Pike, Ark., showed similar relations, asfollows: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 companiesof the 2d Battalion in the proportion of three recruits to two older men, but noless than 167 recruits became sick before the first of the men who had been incamp prior to September 6.¹⁴ Eight recruits died to each man of longerservice.

Analysis of 494 cases of pneumonia made by the laboratoryofficer 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 ofrespiratory disease among recruits comprising a relatively small proportion ofthe strength of the camp, during the months of April, May, and June, 1918.¹⁶

At Camp Lewis, Wash., of 140 cases of pneumonia occurringduring September, 1918, 107 were in men of less than one month's service andof 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 givenherein, only the white and colored can be compared under correspondingconditions. The Philippine Scouts, the Hawaiians, and the Porto Ricans were solargely stationed in tropical or nearly tropical countries and in suchcomparatively small numbers that comparisons are of relatively little value.Suffice it to say that the Porto Ricans showed the highest admission rates forthe total respiratory diseases of any group in the Army, while their death ratewas well below the average.¹ The Philippine Scouts showed relatively low ratesfor admissions and deaths as compared to the Army as a whole, but had a higheradmission rate and a very much higher death rate than those of the whiteenlisted men in the Philippine Islands.¹ The same general relations hold forthe Hawaiians.¹ Owing to the effect of the favorable climate themortality was not high but was higher in these native races than in whitesliving under the same conditions.

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

for every group except the troops in Hawaii. The death ratesare higher in all cases, and usually very much higher. Thus the case fatality ofthe colored is seen to be much higher than was the case with the whites. Morevaluable deductions, however, may be made from the study of the monthly annualrates for admissions and deaths by races in the United States and Europe (seeTables 13-22). If the averages of the monthly annual rates in each group betaken for comparison it is seen that in the United States the total admissionrate for respiratory diseases in the colored troops was slightly lower than thatfor the whites; however, this difference is due entirely to the small proportionof cases admitted with a primary diagnosis of influenza. The cases showingrespiratory symptoms more markedly, the bronchitis and the pneumonias all showeda higher rate for the colored than for the white. It is possible that thecolored recruit was slower on the average in reporting his illness, but inasmuchas the total figure is practically the same as that of the white the moreprobable explanation would seem to be that when attacked, the colored manaveraged a much more severe case than did the white man. In Europe the coloredtroops showed an admission rate over double that of the whites for the sameperiod and much higher than that of the colored troops in the United States. Thehighest admission rates then were those of the colored troopsin Europe, followed in order by those of the coloredtroops in the United States, the white troops in the United States and the whitetroops in Europe. The rates of the latter are probably given too low in thetables quoted inasmuch as a considerable number of cases of respiratory diseasewere reported under the heading "Color not stated," somewhat over24,000 in all, most of which occurred in Europe during themonths of September and October, 1918. The comparison of the case fatalities ofthese cases and of the relative proportion of lobar to bronchopneumonia leads tothe conclusion that most of these men were white. However, if all of these wereadded to the white admissions the relations would not be very materially changedand it has seemed better to deal with the figures as shown.

The relations between the death rates of the different groupsis somewhat different. Here thehighest average rate is that of the colored troops in the United States, 16.06for the months covered by statistics for the colored in Europe. During the sameperiod the colored troops in Europe showed a death rate of 11.78 per thousandper annum. For the 31 months for which we have figures for the white troops inEurope their rate was 3.72 as compared to 7.16 for the whites in the UnitedStates 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 bothas to admission and death rates compared to the recruit, the effect of seasoningon the colored soldier is much less marked, and indeed under the conditions hewas called upon to face in Europe his admission rate washigher than that of the relatively untrained men in this country. The seasoned colored soldier, however, did show a small gain in thematter of deaths, though even here the difference is by no means so marked as isfound in the whites. The effect on case fatalityof length of service as deduced from the figures for the troops in Europe and inthe United States, is to increase the

figure for the white troops and to decrease itfor the colored. It is to beremembered that these conclusions are drawn from the averagesof monthly annual rates and not from the total groupfigures shown in general tabulations. For reasonsalready explained this method is believed to give the more reliable results.

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

In other words, the colored soldier is seen to have beenrelatively more susceptible to the infection in that he contracted it in largerproportions in the preliminary waves, thus acquiring an immunity that served toprotect him against the more fatal wave which followed. A somewhat similarrelation 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 areshown by the figures is the relative proportion of bronchopneumonia to lobarpneumonia. It is well known that clinically the differentiation between thesetwo types of disease is not always possible. Confluent types of lobularpneumonia may produce physical signs indistinguishable from those of lobarconsolidation. Even post-mortem examinations may leave one in doubt. However, aswas pointed out in an earlier paragraph, it was noted early in the war periodthat a large number of cases of pneumonia presented clinically and anatomicallythe characteristics of bronchopneumonia rather than those of lobar type.Granting the impossibility of accurate differentiation in many cases, still itmust remain true that in the observation of thousands of cases the figuresobtained are significant and variations in the proportion of one type ofpneumonia to another between different groups of soldiers, or from month tomonth in the same group, may prove to be important in the study of the effectof race and length of service.

Table 12 shows the ratio of bronchopneumonia to lobarpneumonia for the different subdivisions of the Army for the period of the war.These figures, like those for case fatality, are independent of any strengthestimations and so are strictly comparable. They are, however, summation figuresfor the entire 33 months and hence, since the great majority of the cases anddeaths occurred in September and October, 1918, they more nearly represent thevalues for those months than an average. These figures show that for the entireArmy 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 UnitedStates the corresponding figures were 0.84, 0.61, and 0.39, while for the Armyin Europe, 1.30, 1.19, and 49. It is seen that the values of these ratioscorrespond inversely in a general way withthe relative resistance of the various groups, the officers suffering least fromthe epidemic, the white enlisted men next, the colored enlisted men having thegreatest losses. It is also seen that the corresponding groups show a higherratio in Europe than in this country.

STATE OF NATIVITY

Figures are available showing the numberof admissions and deaths from all the respiratorydiseases according to the State of birth of the patient asgiven by him at the time of his admission into the hospital.¹Fromthese it is possible to calculate directly the case fatality by States. Thequestion of calculating rates of incidence and mortality from these figures iscomplicated by the fact that we have no knowledge of the number of men born ineach State who served in the Army. Comparative rates have been published basedon the total inducted strength from each State and probably in many cases thisresults satisfactorily. However, such rates take no account of foreign-borninductants, of whom there was a large number from some States, nor of the effectof migration from State to State. Certain of the Western States showing very lowrates when treated in this way can be shown to have had in 1918 over three timesas many men of military age as there were children 18 years younger in 1910. Asimilar effect on the rates is produced by the presence from a State of a largeproportion of foreign-born inductants. Manifestly the excess is the result ofimmigration, and to base nativity rates on such an increased number of menresults in a rate far too low. The reverse is true of States losing populationby emigration. In order to obtain figures on which it might be possible to basecomparisons of incidence, it seems best to prorate the inducted strength of theArmy between the States in the same proportion as the States had boys under 10years of age in 1900. This should result in a fair approximation to the numberof men born in each State who served in the Army. Inasmuch as this method takesno account of the foreign-born soldiers the rates are of value only forcomparisons between themselves. The method should allow us to state that thedeath rate in natives of one State was higher than in natives of another, but itdoes not permit of comparisons with other rates based on more complete data.Table 24 shows the number of admissions and deaths for total respiratory diseaseby native State for white enlisted men with rates calculated according to themethod described, and the case fatalities. In the first column of Table 25 theStates have been arranged in the order of incidence rates for white enlisted menbeginning with the State having the lowest rate, while the second column showsthe order with respect to mortality and the third case fatality. It is seen thatthere is a general correspondence between the two columns, though somedifferences 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 UnitedStates showing the relative rates for admissions, for deaths and the casefatality of respiratory disease, the rates calculated as described above. Thedeath rates in particular and to a lesser extent the admission rates are fairlyconsistent, in that neighboring States of similar topography and similar densityof population show similar rates. The extremely high rates of Nevada and the lowrates of Arizona and of Delaware were based on relatively much smaller numbersof cases than was the case for most of the other States and possibly areconsequently less reliable. The fact, however, that in all three of these Statesthe figure for the case fatality falls in the same relative position tends tostrengthen the admission and death rates.

TABLE 25.-Relative positionof the States in respect of rates of natives for admissions, deaths, and casefatality from the respiratory group of diseases, white enlisted men, UnitedStates Army. Arranged in order from lowest to highest. The State showing themean 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 ofthe war there was no such preponderance of disease and death from respiratorydisease among the natives of the Southern States as was brought out by Vaughanand 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 thecountry. It is not noticeable, moreover, that the States with the largest citiestend to have lower rates in both admission and mortality columns. The relationbetween the three series of data is interesting. The variability of the rates isnot the same. Of

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CHART XVII.-Therelative admission rates for the respiratory group of diseases for whiteenlisted men by State of birth

CHART XVIII.-Therelative death rates from the respiratory group of diseases for white enlistedmen by State of birth

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the three series the case fatality shows the least variationbetween the States with a coefficient of variation of 0.16, admission rates weremost variable, coefficient 0.26 and the mortality rates stood between with acoefficient of 0.20. It is evident that the mortality rate must bear a directrelation to both the admission rate and the case fatality. For the series as awhole the coefficient of correlation between the rates for admissions and formortality is + 0.67 ? 0.052. This is a high correlation especially whenthe variability of fatality rates is considered. It is thirteen times itsprobable error. Of special interest is the fact, however, that the coefficientcalculated for the death rate and the case fatality figures is practicallyidentical, namely, + 0.677 ? 0.052. The influence on the death rate ofthe two factors, admission rates and case fatalities, was then about the same.This would be easily understood if the admission rate and the case fatalitycould be referred to the same set of cases. That this is improbable, however, isshown by the fact that there is no correlation between these two series ofrates. The coefficient of correlation between the admission rates and casefatalities is - 0.0962 ? 0.0955. The probable error is almost equal tothe coefficient and the figure is entirely without significance. It appears thenthat two sets of causes, one affecting the admission rate or morbidity and theother the case fatality, were active in determining the differences in the deathrates.

CHART XIX.-Thecase fatality rates (per cent) of the respiratory group of diseases for whiteenlisted men by State of birth

It is evident from study of the outline maps that neitherNorthern nor Southern States, Eastern nor Western, mountainous nor fiat, showedany preponderance of either admissions, deaths, or case fatality. It has been acommonplace observation for years that when numbers of individuals were gatheredtogether from various places and held under common conditions,

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those from cities showed a lower rate of incidence from thesputum-borne diseases than did those from the rural districts. This has beennoted especially for military concentrations of population but also has beenshown to hold for students in universities and other similar assemblages. Thereason for this presumably lies in the greater exposure to contact infection inthe case of the city dweller, and the consequent development of an immunityspecific or nonspecific which tends to protect against invasion by the germs ofdisease. A comparison is possible between the rates of the different States andthe relative urbanity of their population. The United States Census Bureauclasses as rural all communities of less than 10,000 inhabitants. This figure isprobably too high for the purposes of this comparison, but is the only oneavailable for use. Taking the percentage of rural inhabitants of each State asgiven by the 1910 census, which is the median census for the average age for thetroops involved, and comparing the figures thus obtained with the rates foradmission, death, and case fatality we find that between the admission rates forthe war period and the percentage of rural population there is a definitepositive correlation, the coefficient being + 0.362 + 0.083. This coefficient islarge enough to be statistically significant and is over four times its probableerror. It is not a high correlation but in view of the fact that the dividingline between rural and urban is probably too high for our purpose we arejustified in drawing the conclusion that a rural population will show a highermorbidity from respiratory disease when inducted into the military service thanwill an urban one. A similar but smaller coefficient is found for thecorrelation between the rural population and the death rates. In this case thefigure is + 0.311 ? 0.087. When the percentage of rural population iscompared with the case fatality rates, however, all significant correlationdisappears. 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 itsprobable error, deprive it of all significance. It would appear then that atleast one of the factors entering into the admission and death rates is therelative urbanity of the population from which the troops are drawn, and thatthis variable affects the death rates through its effect on admissions and notby any demonstrable effect on case fatality.

If the relative immunity enjoyed by the city dweller wheninducted into the military service be of specific character for the diseasesunder consideration it should follow that the rates for troops should varyinversely with those of home populations if both were exposed to great danger ofinfection. The death rates for influenza and pneumonia for the age group 20-29have been calculated for the registration States of 1918. The specific rate formales of this group is not available from published figures of the CensusBureau, but it is believed that for comparative purposes the combined rate forboth sexes will be equally significant. The correlation between these rates andthose of natives of the corresponding States in the Army during the same periodis low - 0.0639 ? 0.12 and it is less than its probable error. No inversecorrelation is found to exist. The inverse correlation between the Armyadmission rate and the civil death rates for the registration States is slightlyhigher, - 0.2 ? 0.12, but still of such value as to be without statisticalsignificance. That, however, infections of the character of those mentioned aremore prevalent in urban communities is shown by the

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

It may be that the actual fact will prove to be that thenegative correlation between the Army and civil death rates should have shown ahigher figure than that given, as two States, Vermont and Colorado, wereresponsible for nearly half of the plus values in the "xy" column ofthe computation. Both these States had death rates well above the average and inboth instances this was due to an abnormally high case fatality rather than to ahigh morbidity. Inasmuch as the factors leading to high case fatality remainobscure it may well prove to be the case that these States are influencedstrongly by some factor at present unknown which throws them out of alignmentand destroys the correlation. However, there is no justification for throwingthem out of consideration at present, and the only conclusion justified by thesefigures is that there is practically no inverse correlation between the rates ofthe home populations and the natives of corresponding States when serving in theArmy. From this would follow that the civil population had previously developedno immunity to influenza and influenza pneumonias, in proportion to itsurbanity, and that consequently such relative immunity as was shown by therelatively urban soldiers was not of a specific nature. If the civil rates for anonepidemic period, 1913, 1914, and 1915 (same age group), are compared with theArmy rates in 1918, similar results appear. The coefficient of correlation hereis - 0.167 ? 0.134. While the coefficient has the minus sign its sizeand its relation to its probable error are not such as to give it statisticalsignificance. It seems probable, therefore, that the relative immunity enjoyedby the city dweller in the Army was the result of nonspecific rather thanspecific factors. This is of course borne out by the fact that the city dwelleralso shows a relative immunity to such diseases as epidemic meningitis which arenot known to be at any time so prevalent in a population as to induce anyappreciable specific immunity.

Following the suggestion in the work of Pearl,¹⁹who showed that there was a definite relation between the explosivenessand fatality of the influenza epidemic in cities and the total death rate andespecially the rates for pulmonary tuberculosis, organic heart disease, andnephritis, the attempt has been made to correlate the Army death rate with therate for the States in this regard. The death rates for the registration Statesin 1913 for tuberculosis, organic heart disease, and nephritis (age 20-29) werecalculated and coefficients computed. That for the correlation between theserates and the case fatalities in the Army was + 0.179 ? 0.11, acoefficient without significance statistically. The same is true of thecorrelation between the tuberculosis, organic heart disease, and nephritis ratesof 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 theelement of the population whose organic weakness leads to this relation betweenthe rates in civil life was almost entirely weeded out of the Army by theexamining boards. Further the figures are not complete as the registrationStates of 1913 represent less than half of the States of the Union.

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

For the period of the war, then, it can not be said that theinhabitants of any one section of the country showed a marked advantage overthose of any other in the matter of morbidity or mortality from influenza andpneumonia. The evidence in the figures indicates that there are two sets ofcauses acting separately to produce the mortality rates, one acting through theadmission rates and the other through the case fatalities. One of the causestending to increase the morbidity rate is the relative proportion of ruralinhabitants in the States from which the troops come. No general cause has beendiscovered to account for the variations in case fatality between the States. Ithas proved impossible to show any correlation between the rates fortuberculosis, organic heart disease, and nephritis in the States and either theadmission or case fatality rates of the corresponding troops. The fact thatthere was no significant negative correlation between the Army rates and civilrates for the same State during the year of the influenza epidemic isinterpreted to indicate that the civil population had not acquired any specificimmunity to influenza in proportion to its urbanity and that the relativeimmunity shown by men from more urban States was of a nonspecific character.

During the earlier months of the mobilization the relation ofthe nativity to morbidity from respiratory disease was much more marked. In theexhaustive study of the subject made by Vaughan and Palmer¹⁸it was possible to show that the total mortality and, in particular, theadmission and death rates for the pneumonias were much higher in camps that drewtheir troops from the South Atlantic and Gulf States. They show the campslocated not in their geographical position but placed in the center of the areafrom which they drew their troops; also that camps drawing their troops fromFlorida, Georgia, Alabama, Mississippi, Louisiana, and Arkansas exhibited ratesmuch higher than the average. The camps showing the lowest rates drew their menfrom the Northeastern, North Central, Northwestern, and Pacific States. Of theNorthern States, those sending troops to Camp Bowie and Dodge showed the highestrate. These States, Minnesota, North and South Dakota, Nebraska, and Iowa, areall shown to be above the average for mortality for the period of the war inTable 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 thefirst six months of the mobilization. The figures given by Vaughan and Palmer¹⁸show 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 thenegro troops at this time.

During this period the liability of the negro troops tocontract lobar pneumonia was eight and one-half times that of the white troops.²⁰In Alabama there were of military age in 1918 approximately 58 per cent ofwhites and 42 per cent colored.²¹ If thesepercentages contracted pneumonia in the proportion given above, a simplecalculation shows that the white, 58 per cent, furnished only 14 per cent of thepneumonia and the colored, 42 per cent, furnished 86 per cent. There are no dataavailable on which relative nativity rates for the white troops can becalculated for the early months. The nativity tables in the Annual Report of theSurgeon General, United States Army, for the year 1918, do not separate thewhite and colored. However, the conclusion seems justified from the study of thecamps mentioned above that the Southern States showed a much higher morbidityand mortality from the respiratory diseases during the last three months of 1917and for the first three months of 1918 than did the other States and, further, arelatively much higher rate than the Southern States themselves showed for thewhole war period. This relation of their rates will be discussed later and atentative explanation advanced.

If the rates for the war period for groups of States arecalculated 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 alsoshows a line indicating the percentage of rural population in each group ofStates. This chart shows, as did the correlation, that there is some relationbetween the percentage of rural population and the admission rates, but nonebetween the percentage rurality and the case fatality. Chart XXI compares thedeath rates for the groups of States as calculated for the war period with therates for the same sections of the country given by Vaughan and Palmer¹⁸for the early months of the mobilization. It is seen that the greatly higherrates for the rural States shown in the early months did not hold for the wholewar period. It must follow, then, that in the later months when the virulence ofthe influenza epidemic was at its height, the relative rate for the more urbanStates exceeded that for the rural ones.

The figures for the colored troops as regards the effect ofnativity on incidence and fatality from respiratory disease are given in Table26. States furnishing an insignificant number of colored troops are not includedin this table.

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

CHART XX.-Relativeadmission and death rates and case fatalities for the respiratory group ofdiseases in the various groups of States, April, 1917, to December, 1919. Forfacility of comparison, the different rates are drawn to a different scale. Thepercentage of rural population for each group is also shown

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TABLE 26.-Relativeadmission and death rates and case fatality for the respiratory group ofdiseases 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.-Relativedeath rates from the respiratory group of diseases by groups of States for thewar period with the figures given by Vaughan and Palmer for the early months ofthe mobilization. The differences shown in the early months are largelyobliterated in the longer period.

CLIMATE AND WEATHER

Tables 10, 11, and 12 show that the effect of the influenzaepidemic in increasing mortality was felt much less in troops stationed intropical or subtropical climates than in the temperate regions. The effect onincidence of influenza of the warmer weather was much less than upon the deathrate. Comparable groups of troops showed nearly if not quite as high anincidence

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rate in tropical stations as in the United States, but thecase fatality of the epidemic was far lower. The case of the white troops inPanama, with an incidence rate of 166.18 per 1,000 (higher than for the AmericanExpeditionary Forces) and a case fatality of only 0.27 per cent, is a goodexample. The experience of the Porto Rican troops at Camp Las Casas is also tothe point. During the October epidemic 16 per cent of its strength contractedinfluenza, and but 0.52 per cent died. These troops when transported to theUnited States showed at least as high a fatality as the whites with whom theyserved. The experience of the white troops in the Philippine Islands and inHawaii was similar. In these comparisons, however, the element of length ofservice is difficult to eliminate, as in general the troops at these distantstations were more permanent and averaged longer service than the troops in thetraining camps in the United States.

The effect of climate on the incidence and mortality oftroops in the United States, however, is fairly well seen. Of camps of over5,000 strength, 16 were situated in the North and 24 in the South. The incidenceof 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 relationswere 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 followingrelation 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 thatthis relation did not hold in the smaller camps. The greatly larger size of thecamps of the first group thus impresses itself on the totals, and as a whole theincidence of influenza was higher for camps in the northern part of the country.There were, however, notable exceptions even among the larger camps. CampBeauregard, La.; Camp Bowie, Tex.; and Camp Cody, N. Mex., were the three campshaving the highest percentage incidence of influenza, and all three weresouthern camps. The same variable that interferes in so many

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

When the mortality rates are considered, however, theadvantage of the southern camps becomes evident. The corresponding figures formortality 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 thenumber of camps listed in the incidence and mortality tables is due to the factthat in each case only camps reporting complete figures are included. Hence thecamps are not absolutely the same in the two sets of tables. The comparativevalue of the figures is not affected thereby.

From these figures it becomes evident that the mortality fromthe 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 thecomplicating pneumonias, we may say that while a soldier in a southern camp wasjust about as likely to contract influenza during the epidemic as his comrade inthe North, his chances of complicating pneumonia and of death were very muchless.

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

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

The effect of weather on the epidemic is one that isdifficult to estimate. Expressions of opinion by individual officers, sometimeseven in the same camp, are at variance. In general it may be said that theweather 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 thanthe average for some years past, but even a cool September would not be expectedto be cold enough to cause hardship. In some camps rainy weather prevailedduring the epidemic wave; in others fair weather was reported. Of 111 stationsreporting, 89 characterized the prevailing weather at the time of the outbreakas mild, 22 as severe. Of those reporting mild weather, 43 showed a mortalityabove the average of their respective groups, 46 below average. Of thosereporting severe weather, 12 showed a mortality above average and 10 below. Itis evident that weather conditions were favorable, as a rule, during theepidemic and that the mortality was little if at all influenced by severeweather when it occurred.

HOUSING CONDITIONS

It is possible to study the effect of housing conditions on thecourse of the epidemic from two points of view: First, the space assigned toeach man in the barracks, the effect of crowding; second, the effect of the typeof quarters, whether tents or barracks. It is difficult by any ordinary methodsto obtain figures for either of these variables that are clean-cut and are notinfluenced by other factors known to complicate the situation. It is impossibleto estimate with any degree of accuracy the proportion of short-service men inthe different camps and as has been shown a considerable difference in thisrespect would introduce a factor in the comparison that would materially alterresults. The same may be said of the geographical situation if comparisons arebased on mortality though this is less a factor when incidence rates arecompared. However, certain conclusions may be justified and, accordingly, theresults of the study are given briefly.

It is evident from the study of the death rates, reported bythe Census Bureau, that, in general, cities suffered from the influenza epidemicmore than did the rural communities. This accords with the general experience inpneumonia mortality over a number of years. Therefore, it is to be expected thatconcentration of population, affording increased facilities for the transmissionof the virus, would increase the incidence of the disease. Vaughan has dividedthe various camps existing during the epidemic into three groups-those over5,000 men, those between 1,000 and 5,000, and those under 1,000.²³In general it may be said that the larger the command the greater the chance fordissemination of infection and the greater the probability of crowdedconditions. Chart XXII shows the result of his study in this respect. It is seenthat while the size of the camp shows little effect on the incidence ofinfluenza, the proportion of cases developing pneumonia and the number of deathsare greatly affected, the smaller camps showing a much smaller proportion ofcomplications. A more detailed study by comparing not only the arithmetic meansof the camp rates but the medians and modes as well shows that the incidencerate of influenza also was decidedly higher in the larger camps. How much ofthis is due to differences in physical surroundings and how much to the wellrecognized fact that the men in the smaller camps averaged much longer servicethan those in the larger, can not definitely be said.

Data as to the degree of crowding in the various large campsduring the epidemic are difficult or impossible to obtain in reliable comparableform.

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Figures that are obtainable seem to indicate little if anydifference in incidence or mortality between those reporting crowded conditionsand those showing the reverse. Such results, however, are open to the disturbinginfluence of the other variable factors mentioned above. It would seem that theinfluence of this factor is best estimated by comparisons made between differentorganizations in the same camp whose surroundings are substantially the same andwhich show practically the same proportion of recruits. A study of thischaracter was made at Camp Humphreys, Va., during the fall epidemic.²⁴The organizations reported on are divided into two groups-those inexistence some time and those newly formed. The results are tabulated asfollows:

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 perman 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 ofcamps. It is believed that such reports, dealing with otherwise comparablegroups, are of more value than the massed figures from a number of camps. Itseems fair to conclude, therefore, that there is to be expected a definiterelation between the degree of crowding and the amount of respiratory infection.

There appears to have been little difference in influenzaincidence between the tent camps and the barracks camps. If the mortality ratesfor all the tent camps are compared with those of all the barracks camps, thereis a decided difference in mortality in favor of the former. However, as withone exception all the tent camps were situated in the South, while the barrackscamps were about equally divided, it is seen that the climatic differenceelsewhere discussed interferes with the comparison. If all the northern camps beexcluded from comparison it is seen that there is little or no difference inmortality for the two groups. Camps with relatively high and relatively lowmortality are found in both classifications.

MODE OF TRANSMISSION

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

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

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"sputum-borne diseases." Most of the preventivework directed against influenza and the pneumonias has been based on this ideaof the principal method of spread. The obvious fact that infective material isconstantly sprayed into the air by the coughing patient, from which it isequally readily inhaled by those near by, has tended to render us oblivious ofother possibilities perhaps as important. The r?le of the hand in the spread ofthese diseases has been emphasized, particularly by Lynch and Cumming,²⁵and the importance of "hand-to-mouth" routes in disseminatinginfection has received much study. It can not be said that any agreement hasbeen arrived at as to which method is of the greater importance in spreadingdisease. It is entirely probable that both methods play their part in theprocess.

CHART XXII

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

Influence of contact on incidence andmortality, 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 ingroups coming in closest contact with infected individuals.

Vaughan (Warren) showed a similar relation among civilians inBoston.²⁷ His figures showed that"sleeping contact" was over twice as apt to result in infection as theless intimate forms of contact in the family. These figures seem to have somesignificance in the question of the relative importance of the hand and dropletin transmission. Apparently the latter method is more concerned in sleepingcontact than the former. Lynch and Cumming²⁵maintained that the indirect transmission of infection from hand to hand bymeans of infected dishwater in the dipping method of washing mess kits was themajor route of transmission. The figures presented in their report taken fromorganizations of the port of embarkation, Newport News, Va., support theircontention satisfactorily. They have also shown by experimental methods that theroute they suggest is a feasible one. Their conclusions have been criticized,however, on the ground that other factors known to be concerned in the incidenceof the disease, such as length of service, crowding, etc., were not eliminatedin making their comparisons. Other officers have failed to show a similarrelation between their organizations.²⁸AtCamp Jackson, S. C., a group of organizations, carefully using boiling water inthe washing of mess kits but composed of men of short service, showed a verymuch higher incidence rate during the fall outbreak than did anotherorganization of much longer service that took no special care to effect thoroughsterilization of dishes. Both used the dipping method.²⁹Here apparently the element of length of service was far more important indetermining 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 wassufficient to kill the usual organisms found in respiratory infections, such asthe pneumococci

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and streptococci.³⁰ Theseexperiments, however, did show the possibility of the transference of organismsof 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 isnot yet definitely recognized.

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

In spite of all evidence pointing to the importance ofcontact, attempts to transmit the disease experimentally under controlledconditions have uniformly failed. The United States Public Health Servicesponsored two experiments of this character during the fall outbreak in 1918.³¹One experiment was carried on in Boston, with 100 volunteers from the Navy, ofthe 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 frompatient to volunteer. The attempt was made to induce the disease by theinjection of citrated blood from patients and the injection of filtrates ofnasopharyngeal secretions. Finally these men were exposed to the most intimatepersonal contact with patients in wards, all with the complete failure toproduce the disease. A similar experiment was conducted with the same result inSan Francisco. The explanation of this result is lacking. Either the propermethod of transmitting the disease was not used, which seems very unlikely inview of the diversity of methods employed, or the volunteers themselves wereimmune to the disease either naturally or through previous infection in spite oftheir negative history. This latter hypothesis is hardly satisfactory thoughseemingly more probable than the former.

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

As to the duration of the period of infectivity of theindividual case, no reliable deductions may be made. The negative attempts attransmission experiments already quoted suggest the possibility that theinfective period is very short, possibly even limited to the period ofincubation or invasion. This idea is also supported by the observation thatdifferent methods adopted by different commands in the handling of theirinfluenza cases apparently produced little effect on the incidence of thedisease. Some camps attempted

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the immediate hospitalization and isolation of all suspiciouscases, others reserved their hospitals for the seriously ill only, leaving thelighter cases for treatment in regimental infirmaries or in quarters whereopportunities for transmission to the uninfected would appear to be much morenumerous. It is not possible to show that this latter method of handling thesituation resulted in any increase in the relative number of cases. The possibleexplanation may lie in a very brief period of infectivity, limited to the periodof invasion or the earliest hours of the demonstrable presence of the activedisease.

PREVENTION

With the preceding studies in mind, it appears evident that theprevention of the fatal pneumonias that attack armies may be approached from twopoints of view. First, the classical one that pneumonia is a primary diseasedirectly due to the dissemination of the various organisms to which pulmonaryinflammations may be attributed, aided by such well-recognized predisposingcauses as chilling and exhaustion. Second, the point of view developed in thepreceding pages which seems to show that during the period of an influenza cycleat least the great majority of pneumonia cases bear a direct relation to theprevalence of the so-called common respiratory diseases which during such aperiod appear to be definitely influenzal in origin. The second point of view isevidently the one applicable to the period of the World War. The figures citedand the relations shown make the conclusion inevitable that had it been possibleto exclude the action of the influenza virus from the Army, the pneumoniamortality would have been so far less than it actually was as to have been ofvery little importance in the death records of the war. This statement holds forthe months preceding and following the great fall outbreak as well as for thatperiod.

While it may be shown that certain measures tend to diminishthe proportion of influenza cases complicated by pneumonia, the prevention, orlimitation, of the number of such cases remains the fundamental problem in theprevention of such pneumonia as was seen during the war period. This problem hasnot been solved. Before proceeding to a short r?sum? of the various means bywhich the prevention of influenza was attempted during the war, and the attemptto assign to them their relative value, it may be admitted that as far as theexperience of the last pandemic goes, no practicable preventive measures haveshown themselves to be of decisive value. Such measures as have shown some valueappear to serve the purpose mainly of delaying the spread of the infection, oflessening the explosiveness of the outbreak. In a military camp this is anaccomplishment of no small value inasmuch as it serves to reduce greatly thedaily number of admissions during an outbreak, and correspondingly to lessen thestrain on hospital facilities and personnel, with the result of giving to theindividual patient the possibility of better care and increased chance ofrecovery. Preventive measures accordingly should be judged by the measure oftheir ability to prolong an outbreak by the diminution of its explosiveness, aswell as by their ability to lessen the percentage of persons attacked.

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MEASURES DESIGNED TO PREVENT THE ENTRANCE ANDSPREAD OF INFECTION IN A COMMAND

QUARANTINE

Absolute quarantine has been shown definitely to excludeinfluenza. The experience of Fort St. Philip, on the Mississippi River below NewOrleans, is a case in point. This post was able to maintain an effectiveisolation and entirely escaped infection during the fall wave of the disease.³²The San Francisco naval training station, situated on an island, carried4,950 men through the height of the epidemic without a case.³³However, very few stations are so situated as to be able to maintainperfect isolation, certainly none of the size of the great war training camps.Certain camp commanders, recognizing the futility of attempting quarantine inthe face of the necessary supply problem, troop movements, etc., made no attemptto enforce isolation. Others restricted intercourse between their commands andadjacent communities in so far as it was possible to do so. There does notappear to be any significant difference between the two groups of camps thusdivided. 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 anincidence below the average, while 11 of 21 unquarantined camps were also belowaverage in this respect. The respective percentages were 53.5 and 52.4. There ishere no significant difference leading us to believe that such quarantineregulations as proved practicable during the war are of any value in reducingthe total incidence of disease. In the six large camps that did not attempt toenforce 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; CampForrest, Ga., 5 weeks; Camp Humphries, Va., 6 weeks, and Camp Logan, Tex., 7weeks, an average of 4.8 weeks as compared with 4.9 weeks for the 36 large campsfor which information is available.³⁴Thereis no evidence here that quarantine availed to prolong the outbreak and thusdistribute the cases over a longer period. To be of avail in excludinginfluenza, quarantine must more nearly approach perfection than provedpracticable in the large camps of the war period. The experience of certaincivilian institutions too, from which the disease was excluded by quarantineuntil the subsidence of the outbreak, only to have it appear promptly as soon asrestrictions were removed, seems to indicate the futility of general quarantineas a military measure. That certain camps, where great stress was laid uponquarantine, 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 limitingthe disease almost entirely to recruits who brought it with them. The personnelof Camp Wheeler, other than the new men, was very small and was composed almostentirely of men who had passed through the relatively severe spring outbreak inthat camp, when indeed the influenza rates were higher than they were in thefall. The fact that these men were all of long service and had passed throughone well defined outbreak less than six months before is a more probableexplanation of their immunity than the institution of quarantine. On the otherhand, Camp Humphries, Va., which instituted no quarantine, had a lower incidenceof the disease than did Camp Wheeler.

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

It is very doubtful whether any measures takenreduced the incidence of the disease. The quarantine seemed to have no ultimateeffect, but did delay the appearance of the disease in the organizations soisolated. For instance, the veterinary training school of about 3,800 menestablished a most rigorous quarantine and all members of the command had theirnoses and throats sprayed daily with argyrol, consequently they had very fewcases, until October 5, when the epidemic reached a sudden peak and then rapidlydeclined, they being practically free from the disease in one week thereafter.Therefore, it would seem that the only benefit from the measures taken was thatthis camp was not overwhelmed at any one time by the number of sick. The diseasewas spread over five or six weeks, allowing better care of the sick.

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

Of the 34 large camps that instituted quarantine, 23conducted a special medical examination of all new men entering camp with a viewto the detection of infected individuals and their prompt separation from theuninfected; 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 22per cent as compared to the 23 per cent average incidence of the large camps asa whole. It is not evident that these measures availed materially in preventingthe entrance of infection. Although no figures are available to support thecontention, it would appear that physical examination of new arrivals in a campshould tend to diminish the explosiveness of an outbreak, as the principles aresimilar to those that govern in the matter of interorganization quarantine. Theobjection found to measures of this character, however, was the practical onethat in times of epidemic the number of officers available for duties of thischaracter is very small, and the rush of work such as, almost inevitably, toresult in hasty, more or less perfunctory examination. Such examinations are oflittle value and take medical officers away from other, possibly more importantduties.

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MEDICAL INSPECTION

Medical inspection, carefully performed, is of unquestionedvalue in securing early treatment of the sick. It is well known that during anepidemic men are often slow to report for treatment and frequently persist ingoing about their duties for hours or days when actually ill. The better themorale of the troops, especially in the face of the enemy, the greater thedanger of this taking place. It is mentioned in reports from the AmericanExpeditionary Forces that men kept their places in the ranks until forced tofall out, often with fully developed pneumonia. Daily, or twice daily, medicalinspection of the men, taking temperatures in suspicious cases, serves to detectsuch cases early and to reduce the likelihood of serious complications. Thereis, however, nothing to show that the institution of this measure materiallyreduced the incidence of influenza in the camps employing it. Under practicalconditions the difficulty of devoting to this measure the time necessary torender it effective makes its satisfactory application almost impossible. Of the78 camps employing daily inspection of the troops, half were above the averagein incidence and half below. The same results were obtained in the 30 camps thatdid not institute it.²²

USE OF THE MASK

The value of face masks worn by the whole of a command has beenthe subject of much argument. In the first place, if we hold with those whomaintain that the main route of infection is "hand to mouth" most ofthe theoretical value appears to disappear, though it may limit to some extentthe number of times the hand visits the mouth. It appears to be generallyconceded that the use of the mask by attendants on the sick, exposed constantlyto infection, is of value. That the use of the mask was universal among hospitalattendants is a fact. It is equally true, as shown by the Camp Custer figuresgiven above, that such attendants, especially nurses, were attacked in a muchhigher percentage than the average. They were of course constantly exposed andwere undergoing severe strain in the performance of their duties. Under thesecircumstances judgment as to the value of the mask becomes difficult. Reportsfrom the Durand Hospital in Chicago indicated great value in protectingattendants and in preventing cross infection among patients.³⁶

It appears that masks, to be effective, should be of acertain definite thickness of material and that there are differences in thevalue of different materials used for their construction. Reports from CampGrant, Ill., in the earlier months of the mobilization, while confirming theimpression that cross infection may be limited by this means, have shown that acertain critical thickness of gauze must be used to prevent the passage ofbacteria.³⁷Too great a thicknesswas found to result in an uncomfortable mask that in many instances did notallow the free passage of the breath, necessitating the passage of the latteraround the mask rather than through it. The experiments showed that if thenumber of threads in the warp of the gauze be added to the number in the woofand the sum multiplied by the number of thicknesses used, the resulting figure,to insure efficacy, should be at least 300. Experiments carried on by medicalofficers at the Rockefeller Institute for Medical Research indicated

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

Further experiments performed at the instance of the NationalResearch Council tested various mask materials against a dry suspension of B.prodigiosus in air, the suspension being passed through a filter and thebacteria recovered in absorption bottles containing saline.³⁹The latter was then plated out and the count after incubation comparedwith that of the same suspension run without filtration as a parallel control.These experiments showed that the three-layer butter-cloth mask, shown by theexperiments at the Rockefeller Hospital to be efficient in preventing theprojection of infected droplets by the person wearing it, is by no meanssatisfactory in protecting the wearer from the inhalation of bacteria suspendedin the air after the bacteria have lost their original coating of moisture. Indifferent experiments from 44 to 76 per cent of the bacteria passed this maskwhen used dry. It was found that its efficiency was greatly increased bymoistening, and they suggest the use of a mask based on this principle. Thiswork suggests that the care used by many to avoid the use of a mask dampened bythe breath or by perspiration was misapplied. These workers also tested certaingrades 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 sufficientfiltering 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 oflimiting the spread of infection. It is equally evident that unless thenecessary conditions are fulfilled in the construction and wearing of the maskit may be useless and in some cases even harmful. For this reason statistics asto the results attained in the general use of the mask in the Army camps duringthe epidemic are of doubtful value. The classification of the camps in thisrespect 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 generalmasking. However, the considerations given above lead to caution in acceptingthis negative evidence. The expressions of opinions by medical officers aredecidedly conflicting. It is apparent in the consideration of this matter, as inso many others, that the

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preponderant influence on camp rates of the relativeproportions of recruits and longer service men, and, especially when comparingmortality, of geographical position, determines the camp rates and that otherfactors have had relatively little effect. No one has expressed the opinion thatthe mask properly used can do harm. Experimental evidence points to theprobability of its usefulness. Practical experience shows that the necessaryconditions for its proper use are rarely attained except when used by trainedhospital attendants for their own protection. The conclusion is that the mask isprobably of great value potentially but that the difficulties in securing itsproper use by the mass of a command are such as to render its general employmentof doubtful utility.

USE OF PROPHYLACTIC SPRAYS

The use of prophylactic sprays or gargles, not only by newcomersbut as a general measure throughout the camp, was practiced in many commands.The antiseptics used varied greatly. Perhaps the most generally used weredichloramine-T, quinine solutions, and silver nucleinate or argyrol. The valueof the measure and the relative value, if any, of the different solutionsrecommended have not been determined by controlled observations. When the spraywas used in a camp, lightly affected by the epidemic, local opinion wasfavorable; when it failed to prevent a high incidence it was condemned asineffective. There is no general evidence that prophylactic treatment reducedthe incidence of disease in the commands employing it. The figures for the campsfollow: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 incamps using prophylactic methods. While, as indicated above, other factors mayhave been responsible for the difference shown in the two groups, the result issuggestive of possible danger in the use of general spraying. When used on largenumbers of men the danger of conveying infection from throat to throat wouldappear to be very real, especially as the necessities of the case during anepidemic require that this work be delegated to hastily trained personnel. FromCamp Funston, Kans.,³⁴it wasreported that spraying of the nose and throat with antiseptics as a prophylacticfor contacts and attendants predisposed to infection rather than protected. Thiswas proved by a group experiment of 25 attendants on influenza cases who werenot sprayed. One contracted the disease. Of 25 sprayed with protargol solution17 contracted the disease. All 50 attendants wore masks and worked under thesame conditions in the same temporary hospital.

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At Camp Cody, N. Mex., daily spraying was employed in manyorganizations but not in all.²²Itsuse was discontinued upon discovering the incidence of disease to be muchgreater in organizations in which its use was carefully employed than in othersin which it was not used.

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

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

PROTECTION OF TROOPS FROM UNDUE FATIGUE

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

During the summer of 1918 repeated waves of influenza andpneumonia occurred at Camp Funston, Kans., each wave in turn practicallyconfined to recently inducted troops. In one such outbreak a study of the effectof variation in the training schedule was made.⁴⁰Recruits were quartered in two separate parts of the camp, for conveniencecalled A and B herein. Owing to local conditions the amount of drill and fatigueduty in camp A was not over half that in camp B. Owing to congestion at thereceiving station the troops at camp A were held from two to four days beforeundergoing the ordeal of physical examination, inoculation, and equipment.During the first two weeks of camp life this contingent had 3.7 per cent of itsstrength reported sick, while the men at camp B on the fuller schedule reported6.5 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 isbeing called upon for a severe biological effort," that consequently allother effort should be reduced to a minimum, and that recruits should not beexpected to reach a point where hard work on full time is possible for at leasta month.⁴⁰

That the great difference in incidence among recruits ascompared to seasoned troops is not the result of the inoculations againsttyphoid and paratyphoid is the conclusion drawn from a study of this point madeat Camp Funston coincidentally with the work above mentioned.⁴¹It was shown that the curve of pneumonia incidence was a fairly regularone of the usual fre-

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

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

That the Camp Funston figures, showing the effect of fatigueon disease incidence, are probably of general value is indicated by the numerousreports 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 lengthof service. The effect of fatigue and exposure on seasoned troops is seen in thehigh proportion of pneumonias and the high case fatality of the AmericanExpeditionary Forces.¹It seems wellestablished, then, that during the prevalence of respiratory disease in acommand, training schedules, especially those for the newer men, should bereduced to the minimum permitted by military necessity. Indeed it would seemthat 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 largenumbers of men indoors at entertainments and the like.⁴⁴Such a ruling would naturally follow the adoption of interorganizationquarantine, which appears to be of distinct value in slowing the spread of anepidemic. An interesting instance of increased incidence of influenza followingsuch gatherings was recorded at the San Quentin Penitentiary in California,⁴⁵where the weekly moving-picture show was shown to be followed regularly byan increase in the number of new cases of the disease. That gatherings indoorsmay also be the cause of dissemination of the virus is suggested by the factthat on the substitution of outdoor band concerts for the indoor show at thisinstitution the number of cases two days later was still significantly largerthan during the rest of the week. This suggests that the massing of men inclose-order drill may have elements of danger. However, men associated in drillare usually together in mess and barracks, and the drill can hardly be expectedto exert much extra influence.

USE OF THE CUBICLE

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

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are equally applicable to its use in barracks. There is noevidence in the mass statistics that the lack of the use of the cubicles in thefew hospitals that failed to utilize this precaution was of any influence ontheir mortality rates. Nor is there any statistical evidence that screeningbetween beds in barracks lowered the rate of incidence. Indeed, of 19 largecamps using the cubicle in barracks, 12 showed an incidence rate above theaverage to 7 below it; of 19 not using the screen, 7 were above the average and12 below.²²However, if the averagepercentage incidence of the two groups is calculated it is found that the twofigures 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 ofthe cubicle in preventing cross infection in hospital wards.¹³Similar reports were made earlier from Camp Dodge, Iowa,⁴⁶and Camp Taylor, Ky.⁴⁷Areport 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 separatedby sheets. The percentage of pneumonia cases among occupants of these beds was23.36, 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 procuredby the installation of the cubicle system is sufficient to have some effect onthe distribution of bacteria.

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

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

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of these measures is a slowing of the progress of an epidemicrather than any considerable diminution in the number of cases. The differencesin admission rates of different commands depend primarily on differences in therelative numbers of susceptibles, mainly recruits. This being the case, thedevelopment of means of individual prophylaxis or immunization becomes of primeimportance.

PROPHYLACTIC VACCINATION

With the coming of the severe fall wave of the influenzaepidemic, attention was very generally directed to the possibility of individualprotection by means of inoculation of bacterial vaccines. Though reports hadindicated great uncertainty on the part of the bacteriologists as to the primaryetiological relationship of the Pfeiffer bacillus to the disease, most vaccinesused contained this organism. It was usually combined in varying proportionswith type pneumococci, hemolytic streptococci, and even staphylococci. Manyapparently favorable reports were made, but owing to the explosive character ofthe epidemic and its appearance nearly simultaneously in all parts of thecountry, most extensive vaccination experiments were made after the epidemic wason the wane or at least well under way. If, then, it be remembered that the casefatality is greatest during the earlier part of an outbreak, and if the resultsin persons vaccinated relatively late in an outbreak are compared with a controlgroup whose cases and deaths were counted from the beginning, it is seen that itis 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 theorganisms used during the war period in reducing the incidence of the primaryinfluenzal infection may be regarded as negative.

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

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

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prepared at the Army Medical School. It was given in one doseof 1 c. c. The reactions, general and local, were not unduly severe and noserious disability resulted therefrom. The troops vaccinated included both whiteand colored, both seasoned men and recruits. The period of observation followingthe inoculation included the period of the fall epidemic of influenza. Theresults are tabulated below. The vaccination of the older men had beenaccomplished prior to the arrival of the recruits. The latter were inoculatedimmediately 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 ArmyMedical School was tried to a considerable extent on volunteers in other campsfollowing the promulgation by the Surgeon General of a circular letter, datedOctober 25, 1918, authorizing its general use. It was not used, however, until atime when the accurate estimation of results was interfered with by the shiftingof troops incidental to the demobilization. Favorable reports were received fromthe 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 menwas about one-fourth that of the unvaccinated. Camp Custer, Mich., reportedunfavorable results on a small group. On the whole the reports received fromcamps in this country were decidedly favorable. In the American ExpeditionaryForces, a carefully controlled experiment was made at Camp Lusitania.⁵⁰Here, 5,000 men were vaccinated and 3,861 held as controls. Several varieties ofvaccine were used, a lipovaccine containing pneumococci types I and II, and onecontaining all three types, both having been prepared at the Army MedicalSchool, 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 ofobservation January to June, 1919. The results, however, indicated that thelipovaccines reduced the incidence of pneumonia to about one-fourth that of thecontrols. The saline vaccine showed no such result. All vaccines showed areduction in the incidence of influenza and common respiratory diseases, thesaline vaccine in this instance showing as good a result as the others. Acareful series of serological observations was made on representative numbers ofthe vaccinated men. Blood was taken from these men semimonthly for the period ofobservation. Antibodies were demonstrated for pneumococci types I, II, and III,beginning the second week after inoculation, reaching a high point at the end offour weeks, then gradually decreasing during the balance of the period. Theresponse to type I was most marked, type II next, and

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type III least. The sera were tested by agglutination and bycomplement fixation methods. The men inoculated with the saline mixed vaccinegave substantially the same reactions as those on whom the Army Medical Schoollipovaccine was used. Protection experiments with mice also demonstrated thevalue of the treatment.

SUSCEPTIBILITY AND IMMUNITY

During a great pandemic outbreak of influenza the disease isso widespread and affects so large a proportion of the population at one time asto lead very naturally to the impression that practically 100 per cent of thepopulation has been exposed to the disease. If this be so, we must assume thatcertain individuals, perhaps the majority, possess immunity against thisinfection, as the figures do not indicate that the entire population becomesinfected. The figures given in previous pages, which, as stated, probablyconstitute a minimum estimated number of the cases of respiratory disease thatcan be attributed to the influenza virus in the Army during the World War, showthat 26.6 per cent of the men in the military service contracted some form ofthis disease. The reverse of this proportion is that 73.4 per cent, whileequally exposed, escaped infection. It would appear that practicallythree-fourths of the men, living under conditions as favorable to thetransference of infection as can well be imagined, failed to contract thedisease. Does this mean that these men were naturally refractory to thisinfection, that they acquired an immunity at some prior date, or that theyfailed to come in contact with the active virus? That the lost supposition couldbe true to any considerable extent in the Army seems too improbable forargument; it might have held some place in the population at large, where theindividuals are not in such constant contact with each other and particularlyare not habitually associated in relatively large groups. It may be safelyassumed for present purposes that practically every soldier had the opportunityto contract influenza if his physical condition was such as to render himsusceptible to it. Assuming this, then we are forced to choose for ourexplanation of the immunity shown by the majority of the men one of the othertwo possibilities though granting that both may have had their part in producingthe effect. The possibility must be borne in mind that many cases may haveoccurred of such mildness as to have attracted no particular attention, butstill 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 uswhich in many ways is most like influenza is measles. Immunity to this diseaseis rarely congenital. It is generally admitted that practically all persons aresusceptible to it unless protected by previous attack. With measles, however,the immunity conferred by an attack is usually permanent. Bearing in mind theanalogy of measles, we should naturally feel that immunity to influenza was mostprobably acquired and due to a previous attack, but here we find that thequestion of acquired immunity itself has been called into question.

Earlier writers on the subject were inclined to deny theexistence of immunity after an attack of influenza. Parkes⁵¹in 1870 said, "There is some

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discrepancy of evidence; but, on the whole, it seems clear that, whilepersons seldom have a second attack in the same epidemic (though even this mayoccur), an attack in one does not protect against a subsequent epidemic."This opinion has been repeated in substantial agreement by many authoritiessince, and it is a familiar observation that the same individual may haverepeated attacks of "grip" from year to year. Whether these repeatedattacks are indeed due to the same virus is, as has been said before, still opento argument. On the other hand, it seems necessary to assume that convalescencefrom influenza involves some degree of immunity, as otherwise we should be facedwith a condition wherein each susceptible person would contract attack afterattack in rapid succession. Moreover, the usually accepted explanation for thepassing of an epidemic wave, that of the exhaustion of susceptible material,depends on the assumption that those who recover from the disease are at leasttemporarily immune. While we must grant that such immunity in the case ofinfluenza does not last for life, or even perhaps for any considerable period oftime, a number of observations were recorded during the World War that tend tothrow light on this problem and to make possible a fairly definite answer.First, we have the general observation that troops who passed through theepidemics in the winter and spring preceding the great outbreak of the fall of1918 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 outbreaksaffected the Army more extensively than they did the general population. This isa fair assumption, as we have shown that this was true to a considerable extenteven 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 nonspecificimmunizing process, as suggested by the analogy between the effect of serviceand of relative urbanity of troops, as shown above. Hence the relative immunityof the American Expeditionary Forces, for instance, may have been due more to anonspecific seasoning process than to the development in its men of a specificimmunity. If this be true, it is fair to add that the degree of protection thusafforded is probably as great as could be expected from this process. Theaverage admission rate for the American Expeditionary Forces from respiratorydiseases was 143.4 per 1,000, the corresponding rates for the troops in thiscountry was 227.7, a figure about one half again as high as that of the AmericanExpeditionary Forces.¹If we canshow for various units a degree of protection greater than this following aprevious outbreak of the disease we shall be justified in assuming that specificimmunity entered into the case.

At Camp Shelby, Miss., there was in April, 1918, a division of troopsnumbering about 26,000.⁵² An epidemic of mildinfluenza struck this camp at this time, and within 10 days there were about2,000 cases, including not only men who were sent to the hospitals but also menwho were cared for in barracks. This was the only division that remained in thiscountry 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. Itconfined itself almost exclusively to the recruits of the summer and scarcelytouched the men who had

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lived through the epidemic of April. Not only the 2,000 who had the diseasein April but the 24,000 who apparently were not affected escaped the fallepidemic. Vaughan stated:⁵³ "It appearsfrom this that the mild form of influenza of April gave a marked degree ofimmunity against the virulent form of October."

This observation points to the existence of both possibletypes of immunity: A natural type possessed by the body of the above commandthat failed to contract the disease on either exposure, and an acquired type inthose 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 ofmen, was attacked by an epidemic of influenza which involved 613 men in astrength of about 1,200. There were two deaths from pneumonia. Company B, theunit first attacked, had almost all of the cases for the first two weeks, whenthe other companies were also attacked. This regiment thus showed an attack rateof over 50 per cent at this time, the company first attacked showing the lowestincidence. During the succeeding five months, the period of greatest mortalityfrom influenza, this regiment was working in the St. Mihiel and Argonne sectors.About 150 men had colds of varying degree, usually attributed to the conditionsunder which they were living and working. There were 3 cases of pneumonia, ofwhich one died. The regiment thus passed through the worst of the influenzaepidemic with practically no sickness. In early January, 1919, the regiment wasgrouped at Commercy and moved to Bordeaux for shipment home. Here it was againattacked by influenza, then present in the civilian population with a dailymortality of about 1 to 2,000 inhabitants. During January and February therewere 270 cases of influenza in the regiment, with 35 cases of pneumonia and 5deaths.

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

	May and June, 1918, influenza	January and February, 1919
	May and June, 1918, influenza	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 inassuming that the early epidemic had conferred sufficient immunity to keep theregiment free from influenza for six months, and partial immunity extendedthrough January and February." He noted further, that some of the menattacked in the winter had also had influenza in May or June, but as a rulethese proved to be mild cases.

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Vaughan⁵³ says: "Thosewho had influenza in September and October were not affected by the recurrentwaves. In the recurrence of influenza at Dorr Field during January, 1919,Squadron A, which was affected most severely in the first epidemic, had nocases. From Camp McClellan we get a report of a similar incident. Speaking ofinfluenza during December, the report goes on to say that Battery A, which hadvery few cases during the October epidemic, sent more cases to the hospital thanany of the other units in this organization, while Battery G, which was affectedmost during the epidemic, had very few cases this month. Camp Jackson reported arecurrence of influenza in January, which was localized largely in the 48thInfantry, a regiment which had passed through the autumn epidemic with very fewcases at Camp Sevier."

An interesting comparison is reported from Camp Dodge, Iowa.⁵⁵In this case the influence of seasoning, or length of service, may beeliminated. There were in this camp at the time of the fall outbreak, tworegiments of Regular Infantry. The 2d Infantry had been in Hawaii and there hadencountered the earlier wave of influenza, reporting 300 cases. The 14thInfantry, partly from Alaska and partly from Fort Lawton, Wash., arrived in campduring 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 14thInfantry, in 48.5 per cent. They were indeed the organizations having the lowestand highest incidence of all camp organizations, respectively. If thisobservation be of general significance it would point to the fact that seasoningin regard to this disease is a specific rather than a nonspecific process.

These more or less fragmentary observations might bemultiplied almost indefinitely, but enough has been quoted to show that anorganization which had passed through one outbreak of influenza was much lesslikely to suffer as severely a second time, indeed had apparently, for somemonths at least, received a substantial measure of protection, much more thandifference in length of service would imply, especially as in cases like that ofthe Engineer regiment described above the organizations were already seasonedwhen the first epidemic appeared. It would appear, however, that in anyorganization there are many men who do not take the disease in recognizableform, even in a succession of epidemics.

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

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THE INFLUENZAL CYCLE DURING THE WAR PERIOD

It was pointed out above that we are apparently justified inassuming the incidence, during the period studied, of seven separate waves ofacute respiratory disease associated with pneumonia. The first of these, comingbefore the mobilization of the National Army, is perhaps the most doubtful andcareful studies of its characters are lacking. With the advent in December,1917, of the second well-defined wave of this character, attention was forciblydrawn to the situation by the occurrence of large numbers of cases of pneumoniawith a high mortality. These were at first regarded as secondary to the epidemicof measles which was then on its decline, but as the number of cases ofpneumonia continued to increase it became evident that many, if not most ofthem, 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 betweenthis class of diseases and the pneumonias shows to be its due. Indeed, acomparison between the incidence of acute respiratory disease and the casefatality of measles or with the percentage of measles cases developingpneumonia, shows such a marked degree of correlation that it seems more thanprobable that even the post-measles pneumonias were due in large measure toconcurrent infection with the virus of influenza. Chart XXIII shows theserelations graphically. Such a relation would serve to explain the unprecedentedhigh incidence and fatality of measles pneumonia.

It has been shown that groups exhibiting greatersusceptibility to infection by the influenza virus also tend to show a greaterproportion of lobar pneumonia, while the groups more resistant to the primaryinfection show increasing proportions of bronchopneumonia. In the gradualevolution of virulence and invasiveness on the part of the virus by which itworked up to the peak of its activity in September and October, 1918, it isprobably true that in each successive wave of increasing invasiveness itattacked the most susceptible of the soldiers first, and that each wave attackedmen whose resistance could be overcome at that stage of its evolution, leavingthose that recovered temporarily immune from later and often more fatal attacksof the disease. With each wave the relation between the virulence andinvasiveness of the virus, on the one hand, and the susceptibility or resistanceof the population, on the other, determined the incidence and fatality of thedisease.

The pneumonias of the first winter (1917-18) of the periodunder consideration resembled those of the interepidemic period more closelythan was the case later, and the proportion of lobar pneumonias was greater thanin subsequent outbreaks. Bacteriologically, the findings in pneumonia sputa andlungs varied greatly in different camps. Thus in Camp Travis, Tex., the majorityof the winter pneumonias were associated with infection by the hemolyticstreptococcus, while at Camp Wheeler, Ga., which also had a very high pneumoniarate at this time, this organism did not make itself felt until the advent ofthe third or spring wave of infection, the pneumococcus in its various typesbeing 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, thoughexhibiting a considerably larger proportion of the so-called mouth types, TypesIIa,

CHART XXIII.-Acomparison of the variations in the annual admission rates for the totalrespiratory diseases, the case fatality of measles, and the percentage ofmeasles cases developing pneumonia, white enlisted men in 36 large camps in theUnited States, October, 1917, to March, 1919

III and IV, than was the rule in preepidemic times, still showed a muchsmaller proportion of these types than was the case later when influenza hadattained its maximum virulence. The proportion of the fixed epidemic types, Iand II,

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

At this time the high incidence of respiratory diseaseaccompanied by pneumonia obtained among the troops in France as well as in theUnited 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 thanwas the case at home. Our small body of troops in France at this time did notcover as much territory as was the case later and the infection could involvethe whole command more quickly than was possible in the widely separated campsat home. Probably for the same reason the peak was higher, though the durationof the outbreak was shorter than in the United States. In both groups at thistime 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 forcolored troops were also much higher abroad but at this time were about the sameas those for white troops in this country.

Both here and abroad there was a decided drop in the ratesduring the month of February, 1918. Taking any one small group, such as a singlecamp, the interval between waves is seen to be greater than the one month, butowing to the fact that the outbreaks varied in their time of onset andsubsidence in the different camps, sometimes by several weeks, the curve for thewhole is smoothed and the interval between waves, shortened. In March, 1918,there began to be observed in the United States decided epidemic outbreaks ofrespiratory disease that was generally called influenza. Descriptions of thedisease at this time both here and abroad leave no doubt as to the clinical andpathological identity of the epidemic with that which appeared in the fall. Theimmunity later shown by groups which passed through this spring outbreak showsthat the infection was the same. It was, in most camps, explosive in its onsetand it involved a large proportion of the men in each camp attacked, though inthe 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 distinctpeak of acute respiratory disease occurring either in March or April.¹The other camps showed increases but in such a way as to divide their casesbetween the two months. The rates for the concurrent pneumonia present a strictparallelism 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 influenzain considerable numbers of cases. In several camps, Camp Wheeler, Ga., and CampDodge, Iowa,⁴⁴for instance, thecharacter of the secondarily invading organisms showed a decided change, thehemolytic streptococci replacing the pneumococci with increasing case fatality.At this time the fulminating pneumonia, with wet hemorrhagic lungs, fatal infrom 24 to 48 hours, was first observed. This was regarded at the time ascharacterizing the streptococcus at the height of its virulence. Laterexperience showed the lesion to be influenzal. The 24 camps having a distinctpeak 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 campsexhibiting 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 ratesare not closely correlated. If the rates of the camps showing a March peak arecompared with those in which the peak came in April, the following figures areobtained.

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, butmuch lower death rates and case fatality, than camps having a March peak. Thisparallels the relation found in September and October, as will be seen later.The order in which these camps were attacked roughly corresponds to the sequenceof attack in the fall outbreak. Almost all the camps in the lower half of thetable are southern camps. Of the southern camps having high death rates, CampWheeler, Ga., Camp Doniphan, Okla., and Camp Greene, N. C., all had high ratesduring the winter and were evidently composed of highly susceptible material. Inthis wave, for the first time, men from Northern States were seriously affected.

This wave of the epidemic was very much less explosive in the AmericanExpeditionary Forces.¹ The rise after theFebruary remission was less noticeable and the incidence of influenzalinfections with some complicating pneumonias continued well into the summer. Themild character of the disease, together with lack of agreement as to its exactnature led to the designation of "three-day fever," by which it wasgenerally known at that time. The fact that the troops were now widely scatteredfor training purposes doubtless made its spread less rapid.

It is seen, then, that undoubted influenza appeared at this time in bothEurope and America so nearly at the same time as to render its transference fromone area to the other very unlikely. That it was present as well in other partsof 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 Japanesefleet which arrived with several cases on board.⁵⁷During the spring epidemic,

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both here and abroad, colored troops suffered decidedly morethan did white troops. Following the outbreak of March and April there was amarked fall in admission and death rates for respiratory diseases in the UnitedStates. As has been stated above, the disease remained sporadically active inFrance

CHART XXIV.-Therelation between the total respiratory diseases and the pneumonias, annualadmission rates per 1,000 strength for the 24 large camps in the United Stateswhich showed a definite peak for these diseases in the 1918 spring epidemic. Thefigures have been combined so that the peak months in all the camps coincide

throughout the summer. Moreover, in this country there areindications that the disease was not absent but was smouldering, here and thereattacking the susceptible individual. This is shown in the occurrence of manycases of pneumonia, even in June, the month of lowest incidence. Whenever agroup of susceptible individuals was found there was apt to be an influenzaloutbreak

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with a varying proportion of pneumonia. The summer of 1918was one of troop movements on an enormous scale. Camp after camp sent itstrained division abroad, and was filled up with recruits. Before leaving, manydivisions received recruit detachments to complete their strength. At CampFunston, Kans., several such drafts were received during the summer, each onewithin a few days of its arrival in camp being attacked by influenza andpneumonia.⁴¹ These recurring epidemics inrecruits were carefully studied by a special board and a full report waspublished.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. Ithas been shown above that the rates for influenza and pneumonia began to riseearly in August, and increased progressively from that time to the high point inthe fall. It is noticeable that the mortality rate of these cases increasedthrough the warm months of the year, when mortality from respiratory disease isat its minimum.

In France, following the spring outbreak, there was a gradualdecline in the rates for the American Expeditionary Forces as a whole,individual organizations having sharp epidemics.¹July showed the lowest rate for the summer. Observers who investigated thesedifferent outbreaks expressed the opinion that there was a progressive increasein the severity of the disease as time went on. This is well illustrated by theseries of epidemics occurring in the Artillery training camp at Valdahon.⁵⁸To this camp brigades of Field Artillery were sent for special work, oneleaving as another arrived. Early in July, 1918, an outbreak was reported hereand was investigated by officers from the office of the director of laboratoriesand infectious diseases, Dijon. This mild epidemic ran its course in a few weeksand the disease subsided until a new brigade entered the post. Within a few daysinfluenza broke out among these troops, affecting especially organizations thatoccupied the barracks vacated by the companies that had shown the largest numberof cases in the first outbreak. As there was little chance for contact betweenthe troops and the permanent camp personnel, it is probable that the viruspersisted in the barracks. Later the disease broke out in a third brigade inspite of careful disinfection of the barracks, in this instance being somewhatdelayed in its onset. With each outbreak in visiting troops there was anincreased number of cases among the permanent personnel. Troops in neighboringtowns, even one regiment that marched into the camp daily for firing practice,failed to contract the disease.

That this progressive increase of invasiveness and virulenceon the part of the influenza virus was also taking place in the United States isshown by the progressive increase in case fatality during the summer monthsshown in the general tables and charts. An occurrence somewhat similar to theValdahon epidemic took place at Camp Shelby, Miss., beginning August 29, 1918.⁵⁹The 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 asfatal as later waves in the same camp, probably due to the importation of casesfrom the North, but attack at this time afforded protection against infectionwith the more virulent strain. There was a decidedly higher fatality than hadbeen observed in the summer outbreaks. This outbreak and the subsequent wavesoccurring with the advent of new men and practically limited to them are shownin Chart XXI.

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This progressive increase in virulence and invasivenessculminated in the explosive outbreak of the autumn wave of disease which reachedepidemic proportions early in September and reached its peak in the followingmonth. Again the increase in cases seems to have affected both the AmericanExpeditionary Forces and the Army at home at very nearly the same time. Thedifferences in incidence between the two main portions of the Army, that is, athome and in France, are shown in the general tables and charts. In this countrythe negro troops showed a relatively lower incidence at this time than before;indeed their admissions throughout this outbreak and their death rate duringSeptember were lower than that of the whites. The case fatality, however, incolored troops is consistently higher than that for white troops. Both racesshowed higher admission and death rates for the country as a whole in Octoberthan 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 wherethe 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 totransmission of infection from some previously affected point. Camp Devens,Mass., the first to report the highly fatal type of the disease,⁶⁰is supposed to have been infected from the city of Boston. The first cases therewere on the naval receiving ship at Commonwealth Pier, August 28; thence thedisease spread and soon infected the city. It is customary to think of all thesubsequent influenza as having spread from this point and in many cases it ispossible to trace this spread with some definiteness. There are some facts,however, that tend to the belief that there was a general increase of theactivity of an already widely distributed virus which might have given rise tothe fatal wave independently of infection from abroad.

Of these facts the first is that already mentioned, that fromearly August the influenza rates for the Army showed a progressive increase ingeometrical ratio, the curve of rise plotting out on logarithmic paper as apractically straight line. (Chart XIV.) The second is the occurrence of definiteoutbreaks of increasing severity during the summer in this country andespecially in the American Expeditionary Forces. These outbreaks ran right upto, if they did not indistinguishably blend with the great fall wave. The caseof Camp Shelby, Miss., has been discussed. Camp Logan, Tex., is another campwhose outbreak appears to have been entirely independent of the Boston strain ofvirus, at least in its early stages.⁶¹Thedisease appeared here just two days after its advent at Camp Devens, Mass. Thereis no known means by which the infection could have been transmitted from CampDevens, and the disease did not make its appearance in the civil population ofTexas for some time thereafter. The possibility of this outbreak having been dueto a strain of the virus less virulent than that active in the NortheasternStates is indicated by the fact that the fatality at Camp Logan was less thanhalf that at Camp Devens and the further fact that the outbreak is recorded ashaving been of little severity until the receipt of recruits from the Northalready 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 thedisease may have evolved its virulent type more or less generally throughout theworld rather than that the virulent strain arose in one place and spread bycontact, is the apparent impossibility of tracing this spread with anydefiniteness. As stated above, most stations regarded the disease as importedfrom without. In the American Expeditionary Forces it was felt that the severeinfluenza 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 theirown.⁶³This detachment had leftAmerica late in July, 1918, and at a time when the incidence of respiratorydisease was near its lowest here. The outbreak was of a severe type, with a highmortality, and for some time was limited to this detachment. Numerous reports ofoutbreaks in the American Expeditionary Forces during the severe wave expressthe belief that the disease was introduced by replacement troops from the UnitedStates.⁶⁴On the other hand, thegeneral belief here was that the fatal influenza was introduced from Europethrough the port of Boston. It is possible, of course, that neither view iscorrect, but that the fatal strain originated somewhere else and was introducedto both theaters nearly simultaneously. This is suggested by the report that aNorwegian vessel landed several severe cases in New York early in August, havingsuffered greatly from the disease on her voyage; however, there was nosuggestion of fatal influenza in Norway at this time.⁶⁵The fatal type must have evolved somewhere from the less virulent variety. Itsessential indentity with the latter is shown by the facts of immunity alreadyrecorded. It appears possible that this evolution might have occurred in manyplaces at nearly the same time. An observation along this line is the fact thatin several camps in the United States the outbreak of fatal influenza waspreceded for several days by increasing incidence of mild cases graduallychanging to the severe type. The introduction of a fully virulent virus fromwithout 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 itsheight in New England numerous cases of coryza and bronchitis appeared at CampSherman. The picture was not characterstic of influenza, but the condition wasso frequently noticed among patients at the base hospital that isolation wasinstituted and special wards set aside for this purpose. The absence of theusual features of influenza led to considerable comment as to the justificationof such a diagnosis. This uncertainty was abruptly and definitely terminated bythe sudden appearance of large numbers of patients exhibiting characteristics ofclinical influenza. * * * By September 24 the transition had occurred.

Somewhat similar observations are recorded for several othercamps.

These facts, then-the undoubted general distribution of thedisease for months preceding the great outbreak and its progressive increase inprevalence and fatality from early in August, the difficulty of showing with anydefiniteness where the fatal type originated, and the suggestion in some reportsof the development of a severe type locally from a milder strain-render it atleast possible, and even probable, that the severe form arose not as a singlestrain but that many strains acquired increased virulence in different places atabout the same time. The question remains unsettled, and more detailed studiesof

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

Table 27 shows the main facts of the epidemic for the largecamps 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 inone or two camps in order to give an idea of the conditions prevailing and ofthe difficulties faced, and to a great extent overcome, in the care andtreatment of such enormous numbers of very sick men at one time.

TABLE 27.-Influenza andpneumonia. Admissions, deaths, and case fatality rates, for 40 large campsin 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 andpneumonia.

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

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then rapidly declined. The following table shows theadmissions by days for a period of seven weeks, the percentage of those admittedeach day developing pneumonia, and the percentage dying. This classification isparticularly valuable in that the cases admitted on a certain day are thusfollowed to their final disposition, and no allowance for the lag betweenadmission and death is necessary, as is the case when deaths are recorded as ofthe day of occurrence. This table shows the abruptness with which the diseasestruck and gives an idea of the problem thrust upon medical authorities in thecare of such numbers. It is noted that there is considerable irregularity in thefigures for those developing pneumonia and dying. This daily variation isundoubtedly due to the fact that the disease attacked the organizations of acamp seriatim and that certain organizations contained more susceptible materialthan others. On the days when the admissions were mainly from susceptibleorganizations the resulting fatality was high, and vice versa.

Grouping the figures by weeks, an interesting relation isdeveloped, thus:

Epidemic of influenza at Camp Upton, N. Y.Admissions by days, and percentage of daily admissions developing pneumonia, andpercentage 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 theproportion of cases complicated by pneumonia increased, while the case fatalityof the same cases became less. This was not due to any change in treatment or toany discoverable change in the bacteriology of the cases. As the whole serieswas studied by the same men, it is unlikely that differing standards ofdiagnosis have any bearing, as might often be the case in comparing figures fromdifferent sources. Unfortunately, this camp is the only one presenting itsfigures in such a way as

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

The following account of the epidemic at Camp Grant, Ill., isintroduced verbatim. Though Camp Grant stood fourth in the proportion of fatalcases during the epidemic, its proportion of admissions was not far above theaverage. It is thought that this account, written by the camp surgeon, willconvey in a more satisfactory way than any other an idea of the character ofthis outbreak and of the difficulties besetting the course of the MedicalDepartment in combating it.⁶⁸

CAMP GRANT DIVISION SURGEON'S REPORT^a

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

The rapidity with which cases developed duringthe height of the epidemic promptly flooded the base hospital, and it becamenecessary to equip various infirmaries throughout the camp to receive patients.When the housing space in the infirmaries was filled, one or more contiguousbarracks in each area were assigned for the reception of patients. All mildcases were received in the infirmary wards, and if the cases became more severethey were transferred at once to the base hospital. These wards were also usedfor the reception of convalescents returned from the base hospital, who wereheld for about a week for observation before being returned to duty. In additionto the observation and attention given the men in the infirmary wards by thesurgeons of the various organizations, a medical officer of experience wasdetailed as visiting consultant. This officer visited each infirmary daily andgave his advice as to which cases should be transferred to the hospital.

The efficiency of the attention given in thesewards 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 wasparticularly acute on account of the lack of a detention or isolation camp. Itwas solved by granting furloughs to selected men after ascertaining that theirfamilies were able and willing to give them proper care or supervision.

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

The character of the complicating pneumonia during this waveof the influenza cycle differed only in degree from those observed earlier; theatypical pneumonias of the earlier months became the rule now and, especially inthe first few weeks of the outbreak, there were noted cases with pneumonicsymptoms of a fulminating character lasting 24 to 48 hours and showing postmortem a characteristic wet hemorrhagic condition with little or no evidence ofinflammatory reaction. As noted above, a few cases of this type appeared in thespring wave. They will be described in more detail in the consideration of thepathology and symptomatology of the epidemic. In the later weeks of theoutbreaks these fulminant cases became more rare, the fatal cases were ill alonger period of time, and the clinical and pathological conditions observedcorresponded more closely with those seen during the preceding winter andspring. Empyema, so common then, was rarely seen in the early weeks of the fallwave, 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 increasingfrequency. In general, the pneumonia of this period was regarded asbronchopneumonia, though many cases were recorded as of the lobar type. Possiblydifferences in diagnostic standards account for this, but in view of the figuresalready discussed, which show that the proportion of lobar pneumonia increaseswith the group susceptibility, it is probable that different groups actuallyshowed a difference in the distribution of the two types. The epidemiologist atCamp Devens, Mass., commented at this time on the singular fact that while whitesoldiers dead of the disease characteristically showed the lesions ofbronchopneumonia, colored soldiers succumbing at the same time were equallycertain to show a typical lobar lesion.

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

As regards the different methods employed at different campsin dealing with the enormous numbers of sick during the epidemic period, somesurgeons stressed the contagious character of the disease, and every suspiciouscase showing fever was at once admitted to hospital and isolated. Withincreasing numbers of cases, many of which developed pneumonia, thisnecessitated the opening of numbers of barrack buildings as annexes to thehospital, especially for the care of lighter cases and convalescents. Anotherplan was to admit to the hospital only the more severe cases, those showing morethan an arbitrarily designated degree of fever, for instance, the less severecases remaining under the care of regimental medical officers in infirmaries orin quarters. This plan had the advantage of relieving the hospitals to someextent but was open to the objection that cases of the diseases were scatteredamong the troops with resultant liability to further spread. At Camp Travis,Tex., a compromise method was adopted consisting in opening a large annexcapable of caring for several thousand cases, under the care of a speciallyorganized medical and nursing staff, the annex to be used for the isolation andcare of uncomplicated cases, patients developing pneumonia being promptlytransferred to the wards of the hospital proper.⁷⁰Though this method would appear the most satisfactory on theoretical grounds,and Camp Travis showed a very low fatality with a large number of cases ofpneumonia, consideration of the results in general shows no particular advantagein any of the methods of handling the situation. If, as suggested, later studiesshould show definitely that the infective period of the disease is mainlylimited to the stage of invasion, the explanation of the failure of all thesemethods to reduce the incidence of the disease will be at hand.

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In correlating the epidemiological facts recorded above itmay be said that they harmonize with the following conception of theepidemiology of influenza. First, the virus of the disease has a distincttendency to periodic increases in invasive power, each such increasecorresponding to a wave or outbreak of the disease. Between such waves thedisease is not entirely absent, but attacks only sporadically. In addition tothese changes in invasive power there are changes in the virulence of theinfection, once invasion is accomplished. This fluctuation in virulence, ifperiodic, is of a much longer wave length than that of the fluctuations ininvasiveness. The apex of such a wave of virulence corresponds to the outbreakof highest fatality in the course of the cycle. Though the virulence is measuredby the proportion of cases developing secondary infections and by the fatality,the degree of invasiveness determines the number of cases attacked in a givenoutbreak. The disease itself is a comparatively mild one seldom fatal in theabsence of definite secondary infection.

This conception would lead us to believe that the earlierwaves of an influenza cycle would attack mainly those groups of individualsespecially 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 affectpersons of greater resistance, each wave leaving its quota of immunes who arenot attacked in later waves. When the invasive power is at its greatest degreeof evolution persons most refractory are attacked.

Parallel to the increasing invasive power runs an increase invirulence as manifested by increasing proportions of complicating pneumonia anddeaths. In the earlier waves the pneumonias found correspond more nearly tothose seen in the absence of influenza, their fatality is less and thecharacteristic hemorrhagic lesion of the influenza lung is rarely seen. Eachcase is of longer duration and the spread of infection from the lungs resultingin empyema and other complications has time to manifest itself before the fatalissue. The anatomical type of pneumonia undergoes a change as more resistantgroups are attacked, the lobar pneumonias becoming more rare andbronchopneumonia more frequent.

In the earlier waves, groups that are of known susceptibilityto the disease were attacked in greater numbers; the fact that the figures forthe whole war period show no such marked relation indicates that these groups ofsusceptibles were early attacked and to a large extent became immune, the lesssusceptible elements being affected later in greater proportion so that in theend there was little demonstrable difference in the incidence in the differentgroups.

After the peak of the outbreak of the cycle was passed thepersistence of the infection was manifested for some months by the occasionaloccurrence of typical cases. When the next increase of invasive power manifesteditself there was again a certain amount of susceptible material for the virus toattack, because of the short duration of the immunity conferred by the earlierwaves, or, perhaps, the fact that possibly not all susceptible individuals, evenduring a great pandemic outbreak, come into effective contact with infectivecases of the disease.

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ETIOLOGY

The experiences of the World War have tended to confuserather 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 theorthodox conception of the etiology of influenza was that of Pfeiffer, whoseconclusions 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 becameevident that the bacillus of Pfeiffer was not uniformly present in the casesexamined. Certain laboratories reported a high percentage of positive results;others, apparently equally well qualified and equipped, failed entirely toisolate the organism or found it in only a small percentage of cases. Doubtsarose as to the specific relation to the disease of the hemophilic bacillus, andit was recalled that this organism had been found frequently during theinterepidemic period associated with other diseases-notably measles, whoopingcough, and pulmonary tuberculosis-and indeed in persons apparently free fromdisease. A difference of opinion arose among bacteriologists which stillobtains. The development and general use of special media favoring the growth ofthe Pfeiffer bacillus, such as heated blood agar, or media tending to inhibitgrowth 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 wasfound also that these media increased proportionately the number of positivefindings among noninfluenzal individuals. The demonstration by Park and hisassociates that the various strains of B. influenz? isolated from epidemiccases in New York City were not serologically identical and indeed had littletendency even to fall into groups when subjected to agglutination and absorptiontests with specific immune sera, was interpreted by many authorities to indicatethat the hemophilic bacilli constitute a group of serologically heterogenousorganisms rather than a single species, and that as an epidemic must necessarilybe due to the same cause in any given outbreak, it is impossible to assign tothe 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 thePfeiffer bacillus or presents some other organism for consideration as the causeof the disease, opinion on the subject will remain unsettled.

Various investigators have suggested a number of otherorganisms as the cause of influenza. The one which has received the mostattention and which has been found in outbreaks not only in this country butalso in Europe is a Gram-positive diplostreptococcus producing a greencoloration on blood agar and seeming to partake of the characteristics of GroupIV pneumococci and of hemolytic streptococci of the alpha type, if not actuallya mixture of these organisms. All of the objections to the acceptance of thePfeiffer bacillus as the cause of the disease apply with equal or greater forceto this apparently heterogeneous group.

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

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pneumonia were due to pneumococci of the so-called fixedtypes. It soon was proven, however, that not only were the pneumococci of themouth types found in a much greater proportion of cases than was expected butalso that other organisms, notably the hemolytic streptococcus, were associatedwith pneumonia in large numbers of cases. The technique of typing thepneumococcus was new to many laboratory workers, and the shortage of micerendered it often necessary to fall back upon the blood-broth culture method ofAvery, which has the disadvantage of failing to distinguish between virulent andavirulent organisms. However, the compilation of available figures from themonths before the fall (1918) epidemic of influenza shows that 1,903 cases werereported from 29 different sources.⁷¹ Thesecases may be tabulated as follows:

Types of pneumococci reported from the camps inthe 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 betweenthe true and atypical types II it is probable that the proportion of the formershould be considerably smaller and the latter correspondingly larger. About athird of the cases were charged to the strictly epidemic types of thepneumococcus. There were differences between camps in type distribution that canbe accounted for neither by the care exercised nor by methods employed. Itfrequently happened that the same camp reported striking changes in theproportion of the types found from month to month.

As time went on and workers gained proficiency in the methodof typing it was expected that the proportion of the fixed types would tend toincrease. As a matter of fact the reverse was true. Reports on the types foundduring the months of September and October, 1918, show a still greater incidenceof the mouth types of pneumococci.⁷¹ Atabulation 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 grouppredominant but other organisms were encountered in association with them invarious places and at different times during epidemics of pneumonia. Of theseassociated organisms, the most important was the hemolytic streptococcus. Thisorganism appeared early in some camps as the exciting cause of postmeaslespneumonia, more fully treated elsewhere. Not all camps having a high incidenceof pneu-

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monia after measles were able to attribute it to thisorganism. The facts with regard to the postmeasles pneumonia at Camp Wheelerhave already been stated. Camp Travis, on the other hand, at the same time had alarge proportion of these cases due to streptococci. It would appear that giventhe necessary predisposition in the form of primary infection, the actualexciting cause of the pneumonia will depend upon the predominance of oneorganism or another in the environment or upon the mucous membranes of thepatient himself. This idea is supported by the fall outbreak in which moststations reported their pneumonia as of group IV pneumococcus origin but someattributed all their fatalities to the hemolytic streptococcus. During this timereports attributed pneumonia to still other of the usual flora of the mouth suchas 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 hasbeen shown. If, as seems certain, this is a true causative relationship, theinfluenza acting as the predisposing cause which rendered the pneumonicinfection possible, it would appear that the earlier, less virulent waves of theepidemic were complicated by the more invasive of the pneumococcus types while,as the influenza itself became more virulent, it further reduced the resistanceof its victims so that the ordinary mouth types were increasingly able toinfect, and also such ordinarily inactive organisms as the staphylococcus, thehemophilic bacilli, etc. The hemolytic streptococcus, and organism of lowinvasive power combined with high fatality, invaded those camps first where forunknown reasons it happened to predominate. Later with the general interchangeof troops between camps it became more widely disseminated. So far as therecords show there is no instance of an outbreak of pneumonia due to thisorganism that was not coincident with an outbreak of influenza or of some acuterespiratory disease that might readily be regarded as of influenzal nature.

It has proven impossible to estimate the number of cases ofprimary pneumonia that occurred in the Army during the war and it follows thatno 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 earlyin the war period that some influence was at work tending to the production offindings at variance with the usual experience in the pneumonias of theinterepidemic periods. That this influence was, at least in great part, thepresence and activity of the increasingly virulent infective agent of influenzahas been shown with some degree of probability. The nonfatal character ofuncomplicated cases of influenza renders a description of the anatomic changesproduced by that disease alone an impossibility. The autopsies were necessarilyall held on the cases of men who had suffered fatal pneumonic complications.This situation renders it necessary to consider the subject of pathologicanatomy from the same standpoint as that of epidemiology. The pathology found incases of influenzal pneumonia will be described and the attempt made to deducethe changes in uncomplicated influenza, at least of the more severe type, byfirst, noting those

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

The sources of information drawn upon for this study arevarious. The unusual character of pneumonia very early led to careful studies onthe part of hospital staffs, and to the publication of numerous reports. Aspecial board of officers studied the condition at Camp Travis in the latewinter of 1917-18, and later made observations at Camp Dodge, Iowa.⁷⁴Another board worked during the summer mainly at Camp Funston, Kans., andduring the fall wave of the epidemic at Camp Pike, Ark.⁷⁵Special reports of the pathology during the last-mentioned outbreak havebeen published by officers at a large number of camps. An especiallycomprehensive study of the subject was made at New Haven in connection with theArmy Laboratory School at Yale University.⁷⁶Reports of the conditions found at autopsy in our forces in France were receivedfrom practically all large centers in response to a circular letter from theoffice of the director of laboratories and infectious diseases, dated December17, 1918. Material from all these sources has been utilized in the attempt todraw a comprehensive picture of the changes found in these pneumonic cases, notonly during the great fall epidemic but in the earlier months as well.

It is stated elsewhere that clinicians found nothing in thepneumonias of the months before late November, 1917, that struck them as beingdifferent from the findings of the interepidemic period in civil life. With theoccurrence late in 1917 of the definite wave of acute minor respiratorydiseases, which was in general coincident with the declining period of the greatmeasles outbreak, the atypical forms of pneumonia began to make their appearancein increasing numbers. Cases in increasing proportion were recognized by theclinicians as bronchopneumonia rather than lobar pneumonia, and this proportionwas still further increased by the results of post-mortem examination. Briefly,the findings at this period were characterized by their variability. In anygiven camp there was a tendency for one finding to predominate, but in no reportis it shown that the pneumonias examined ran true to any one type in even thegreat majority of cases. The details found appeared to be somewhat dependent onthe nature of the bacteria associated with the process. But there was everywhereconsiderable diversity in the bacteria found. Two outstanding facts were easilynoted at this time: First, the fatal cases were characterized by a very largeproportion of secondary or metastatic invasions of the serous membranes,particularly the pleur?, producing highly fatal empyemata; second, the highincidence of pneumonia was largely confined to camps which were made up of menfrom the States of the south and southeastern part of the country, States whosemen had been shown by Civil War figures to be far more susceptible to therespiratory diseases in serious form than those of the north and west.

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

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in which the infection appeared to spread through thebronchial walls for some distance in the interstitial tissue of the lung,producing peribronchial nodular consolidation; lobular pneumonia, in whichgroups of lobules showed consolidation due to filling with polynuclearleucocytes; and mixed types showing two or more of the varieties described. Theinterstitial and lobular varieties were associated with infection by hemolyticstreptococci and the lobar type usually with pneumococci. The influenza bacillusof Pfeiffer was found in a considerable proportion of cases but was not regardedas of primary importance.

Reports from Camp Funston at this time showed substantiallythe same conditions.⁷⁷In both camps(Travis and Funston), as indeed wherever fatal pneumonia prevailed, theincidence of suppurative complications, especially empyema and, to a lessextent, peritonitis and pericarditis, was high and was responsible for a largeproportion of the fatality. At Camp Funston in particular, a form of mediastinalabscess, "subcostosternal abscess" was noted post mortem with greatfrequency. These serous membrane suppurations were commonly of streptococcusorigin. They occurred, however, in pure pneumococcus infections, though withless frequency. In general it appears possible to make the statement that atthis time, of the camps showing a high incidence of pneumonia, those with thegreatest fatality showed the highest proportion of the lobar type of thedisease.

Reports of the pneumonias occurring with the spring (1918)outbreak of influenza showed an increasing proportion of the types ofbronchopneumonia, though typical lobar pneumonia still appeared. At this timethe number and character of influenza cases forced attention to this disease atmost camps. On the other hand, at Camp Dodge, Iowa, for instance, while themedical authorities there reported the coincidence of large numbers of cases ofmild tracheitis,⁷⁸McCallum,⁷⁹who studied the outbreak, stated that the majority of the cases could notbe shown to be related to any recognizable previous disease. This shows thateven when looking for such an occurrence it is possible to fail to recognize theexistence of the primary infection. That the reported cases of mild tracheitisat Camp Dodge were in fact influenza, and the pneumonias of that period wereinfluenza pneumonias will hardly be questioned at this time in view of theexperience of other camps during the same months and the subsequent evolution ofthe pandemic. If more definite proof were needed, it may be found in thedescription of the pathology of the pneumonias studied. In general the findingsparallel those of the previous winter at Camp Travis. Certain changes are notedthat appear very significant in the light of later events. The proportion oflobar pneumonias was smaller, as was that of the interstitial type ofbronchopneumonia. Lobular pneumonia predominated, often confluent. Serousmembrane infections, while frequent, were present in a smaller proportion ofautopsies, as patients were more apt to die before these complications had timeto develop. "At necropsy there was found the most intense congestion of theentire respiratory tract." The bronchi "were markedly dilated towardthe periphery; their mucosa was deep purplish gray." "Great hyperemiaand edema of the bronchial wall is seen accompanied by a less evident newformation of connective

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tissue cells. The adjacent alveoli so far as they are notfilled with red blood corpuscles, are packed with mononuclear cells and densefibrin. The alveolar walls are infiltrated with mononuclear cells and arewidened." Large necrotic areas were noted, eventuating in abscess formationand containing enormous numbers of streptococci. Perhaps most interesting inevidence of the influenzal character of the disease was the occurrence of thehyaline change in the lower third of the rectus abdominis (Zenker'sdegeneration) in two cases with rupture of the muscle and hematoma. All thesepoints are so characteristic of the pneumonias of the fall outbreak,unquestionably influenzal in nature, that their occurrence at this time in thepresence of the epidemic of "mild tracheitis" serves to clear up thecharacter of that infection.

With the passing of this wave of infection the type ofpneumonia recorded was predominantly lobar until the advent of the highly fatalwave in September. The following brief account of the findings in the latterwave, during which the influenza manifested itself at the height of itsvirulence, is necessarily a composite, culled from many published andunpublished articles and reports. In the interpretation of the findings withregard to the part played by the influenza virus in the production of thepicture as well as that played by secondary invaders it is necessary to considerin particular two factors with regard to any given case. These are the durationof the disease before the fatal ending and the stage of the epidemic wave atwhich the onset of the disease occurred. In the consideration of the variedpathology of the epidemic it is necessary to divide the cases into certaingroups which are fairly difinitely correlated with the variations in the twofactors mentioned.

The first group of cases showing best, post mortem, thelesions which will be interpreted as characteristic of the primary influenzalinfection, has been called the "wet, red lung" type, or hemorrhagicpneumonitis. 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 orthree days. A second group, comprising a majority of deaths during an epidemic,showed various pneumonic lesions on a hemorrhagic background. These occurred incases of somewhat less initial severity than those of the first group and lifewas prolonged sufficiently to allow the lesions of secondary infection to fullydevelop. These cases was found in greatest proportion during the height of theoutbreak and showed an average duration decidedly longer than those of thepurely hemorrhagic type, even as much as ten days or two weeks. The type ofconsolidation encountered in this group varied greatly according to the locationof the troops involved, according to race, and possibly to other factors.Extrapulmonary suppuration, such as empyema, was rare, although fibrinouspleuritis or pleuritic effusion were often reported. Still later, in anoutbreak, a third type of cases came to autopsy. These were usually cases ofstill longer duration and in them the hemorrhagic features were much lessnoticeable. So also the acute pneumonic consolidations were not so constant, butevidence of their former presence was at hand in the shape of unresolvedpneumonia, organization of exudate, pulmonary suppuration, interstitial or inthe form of abscess, and secondary suppuration of the serous membranes. Thislast group then resembled most closely those of the previous winter, those ofthe 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 springoutbreak by the rare cases of the type already mentioned. These different typesmust be the result of the interaction of the virulence of the virus on the onehand and the degree of resistance of the persons attacked on the other. As bothfactors are variables all types of pneumonia were produced during the evolutionof the influenza cycle and in any one outbreak graduations from one type toanother were encountered and mixed types were frequent. Bearing in mind theabove facts of distribution we may continue to the more detailed description ofthe types of pulmonary lesion found.

The wet red lung or hemorrhagic pneumonitis gave a picturealmost pathognomonic of acute influenzal pneumonia. The only comparable findingsare those of pneumonic plague and those seen in acute death from toxic gas. Onopening the thorax the first point noted was the almost total failure of thelungs to collapse. On removal the lung retained on the table its natural sizeand shape. The pleur? usually contained a little blood-tinged fluid, rarely anyconsiderable amount. The pleural surface was usually smooth and glistening,though a thin layer of fibrin over areas of greater density was not rare. Thepleural surface of the lung was brilliantly mottled throughout with differentshades of red, from the pale pink of emphysema through the bright color ofrecent hemorrhage to the deep purple of venous blood. This mottling was lobularin distribution, contrasting colors often showing in adjacent lobules. Therewere usually considerable areas of definite emphysema, especially along theanterior borders. The posterior portions, especially of the lower lobes, showedthe darkest coloration, but the changes described usually involved all the lobesto some extent.

On section of such a lung, large quantities of bloody serumescaped from the cut surface, usually containing small bubbles of air. It seemedimpossible to dry the surface by scraping. Portions cut from the lung usuallybarely floated in water; some sank. There was seldom any distinct nodular feelto the lung, though some portions were evidently more solid than others and thecut surface of such areas presented a meaty appearance very similar to that of arecent hemorrhagic infarct. The entire tracheobronchial tree was intenselycongested, of a deep velvety red, spotted here and there with foci of a darkeror more intensely red color. The bronchi contained a thin seromucousblood-stained fluid and the bronchioles, especially peripherally, were distendedand prominent. Rupture of these distended bronchioles was associatedoccasionally with interstitial emphysema, usually confined to the lung, but morerarely becoming generalized subcutaneously. The extent to which the lung wasinvolved in this hemorrhagic and edematous process was at times incredible andwas plainly of itself incompatible with life. The condition wascharacteristically diffuse involving as a rule all the lobes to a greater orlesser extent.

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

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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 samegeneral 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 oftenrupture. 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 belimited to the trachea and bronchi so far as pulmonary extension is concerned.)Wherever sections of the lung are taken for examination the conditions found aremuch the same. There is a general edema. Subpleural spaces, interstitial tissue,alveolar walls, and alveoli are all involved in this process. The characters ofthe exudate are best seen in the alveoli where it sometimes appears as ahomogeneous mass resembling colloid, sometimes as finely granular material, andsometimes containing strands of fibrin. Varying numbers of red blood cells arepresent in this fluid from a scattered few to densely packed massesindistinguishable from a recent infarct. In some cases large numbers of bacteriaare found throughout; in others careful search fails to reveal their presence.In these early cases polynuclear leucocytes are conspicuously absent. Slightlylater, 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 bronchiolarepithelium and which may extend beyond the epithelium into the walls of thesestructures and kill en masse the walls of the alveoli is a lesion which does notoccur in other types of pulmonary infection." In influenza it is the lesionof characterization. In addition the aplastic reaction of the lung characterizedby edema, hemorrhage, and lack of leucocytic response produce a pathologicpicture in these fulminant cases that will hardly be confused with that of anyother disease. Certain details have been emphasized by different students of thecondition which deserve mention. Le Count⁸⁰called attention to necrosis of the walls of the pulmonary capillaries. Wolbach⁸¹regarded a peculiar distribution of a hyaline fibrinlike substance on the wallsof distended alveoli as characteristic of the disease. Most reports mention atendency to hyaline thrombi in lymphatics and in smaller arteries and veins. Thebroad general characterization of the process, however, as showing evidence of atoxic injury of the tracheobronchial epithelium, the process extending entirelythrough the walls of these structures and accompanied by an aplasticinflammation of the pulmonary parenchyma with marked generalized edema andhemorrhage, suffices to define the findings.

The findings in later cases show various types of pneumonicconsolidation on a background of varying degrees of the condition justdescribed. In these cases the power of reaction on the part of the tissuesappears to have been at least partially restored. Lobar, interstitial, andlobular forms of pneumonia, with exudation of polynuclear leucocytes and oftenfibrin into the consolidated areas, characterized this group, the appearancesdiffering from those found in the earlier months by reason of the greaterprominence of the hemorrhagic and edematous background upon which theinflammatory process was superimposed. The existence of the true lobar type ofconsolidation during this epidemic has been questioned by many but the followingstatement on the subject from the report of the Camp Pike commission shouldprove conclusive:¹³

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The pulmonary lesion has been designated lobarpneumonia when it exhibited the well-known characters of this lesion, namely,firm consolidation of large parts of lobes, coarse granulation of the cutsurface, fibrinous plugs in the bronchi, and on microscopic examinationhomogenous consolidation and fibrinous plugs within the alveoli.

This commission found lobar pneumonia in almost half theirautopsies. Everywhere various combinations of the different types of pneumoniawere found and in different localities different types predominated. It wouldappear that, in general, the lobar type of the disease was associated withpneumococci, the interstitial type with streptococci, the lobular type withstreptococci or the Pfeiffer bacillus, and a type characterized by smallperipheral abscesses with the staphylococcus. Exceptions to these rules are sonumerous, however, that definite conclusions may not be drawn from theanatomical findings. Pneumococci may be responsible for either lobar orbronchopneumonia. An especially interesting point, perhaps bearing on thepathogenesis of these conditions, is found in the report of the Camp Pikecommission.¹³ Cultures, post-mortem, were takenfrom the bronchi, lungs, and heart's blood. In cases of bronchopneumonia ofpneumococcus origin the percentage of positive findings in the bronchiconsiderably exceeded that in the blood while in pneumonias of lobar type thereverse was true. This may be evidence of a different route of infection in thetwo types.

A detailed description of the various lesions observed inthese pneumonias is not in place here. Suffice it to say that as the casesoccurred later and later in the epidemic their type more and more nearlyapproximated those of the earlier months, showing increased pneumonicconsolidation and less prominence of the hemorrhagic and edematous condition.Secondary suppurations became more frequent. The same characters were found inthe pneumonias of the recurrent waves, including that of the early months of1920.

It appears impossible to attempt a statistical analysis ofthe variations of pathologic type between different stations or geographicaldivisions. The impression is gained from the study of numerous reports that thelobar type of the disease was more generally reported from the southern campsthan from the northern. The reports of the hospital centers in France whileshowing the same diversity of findings, indicate the relative infrequency oflobar pneumonia among the troops in that country. The reports seldom dividetheir cases according to race, but that the same preponderance of lobarpneumonia in the negro that is noted in the admission and death rates also heldin the postmortem room is suggested by the comment of the epidemiologist at CampDevens already quoted.

While, post mortem, the pulmonary lesions assume commandingimportance, from the epidemiological standpoint, the lesions of the upperrespiratory tract are perhaps of greater significance. The nearly constantpharyngitis is emphasized clinically. Involvement of the accessory sinuses ofthe nose, especially the sphenoidal, was reported with great regularity bypathologists who looked for the condition. The early conjunctival inflammationobserved clinically is perhaps related to the upper respiratory tract infectionor may possibly represent the atrium of infection.

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

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in the adrenal recorded in several reports. This appears tohave been of a hemorrhagic nature, with loss of the lipoidal contents of theparenchyma cells. Some observers have connected this lesion with the prostrationso characteristic of the disease. Of other changes, particularly emphasized inthe reports, the occasional occurrence of interstitial emphysema, in some casesinvolving a considerable part of the body surface, and the hyaline degenerationrather commonly observed in the lower third of the rectus muscle with frequentrupture and hemorrhage, have already been dwelt upon. The degenerations of theparenchymatous organs, though often extreme were in general those seen with anysevere toxemia, although focal hemorrhage apparently due to lesions of thevessel walls were not uncommon. Of suppurative complications, seen mainly incases of relatively long duration, empyema, peritonitis, pericarditis, andmeningitis occurred with varying frequency.

ROENTGENOLOGY

The result of systematic examination of cases of influenza andpneumonia by means of the X ray has been exceedingly valuable, not only inenabling the clinician to greatly increase his knowledge of the progress of thedisease in the individual patient, but also in throwing much light on the natureof the early changes taking place in the lungs, especially in cases ofuncomplicated influenza in which other methods of exploration of the chest areimpossible. Thus the subject of roentgenology might be considered appropriatelyeither under the heading of clinical medicine or under that of pathology. Itseems best to consider the X-ray findings themselves with occasional comments asto the interpretation that seems justified or the special clinical value of theobservations.

One of the most important questions put to theRoentgenologist for answer was whether or not early uncomplicated cases ofinfluenza show any demonstrable pulmonary changes. The majority of cases give noevidence of such involvement on physical examination, or at most a few scatteredr?les usually interpreted as bronchitis. The important question was: Does thegeneralized hemorrhagic and edematous condition found post mortem in fatal casesexist in uncomplicated influenza cases without pneumonic consolidation; and ifso, to what degree? The answer to this question is important in theinterpretation of the post-mortem pathology, in the study of the pathogenesis ofthe complicating pneumonias, and consequently in the development of means ofpreventing these complications. In several hospitals careful X-ray studies weremade and reported, which appear to answer this question quite definitely. Themethod adopted was to take daily, serial plates from a large number of casesfrom admission to recovery or death. In some cases control plates, takenprevious to the onset of the disease, happened to be available. The results ofthese studies on cases, clinically without signs or symptoms of pneumonia, aresummarized in the following quotation from the report of two investigators atWalter Reed General Hospital:⁸²

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

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

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

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

Certain cases of average severity, however, showed onphysical 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 tothe physical signs in practically all of this group. This finding is probably tobe interpreted as an early or slight example of the lesion described from WalterReed General Hospital as "hemorrhagic pneumonitis." This condition wascorrelated definitely with the hemorrhagic infarctlike consolidations discoveredpost mortem. It showed on the plates as a denser, smoky opacity obscuring thebronchial outlines, beginning at the hilus and extending peripherally. It mayinvolve all the lobes, but is seen most often in the lower. This shadow differsfrom the shadows of pneumonic consolidations both in distribution and in lesserdensity. Unless of considerable extent this condition gives no physical signsother than the persistent r?les mentioned; however, when its increase in extentreaches the surface, the signs produced are those heard early in pneumonicinvolvement and before classical signs of consolidation are established. Not allcases showing this type of consolidation eventuate in definite pneumonia, but incases showing such involvement of a large proportion of pulmonary tissue theusual outcome is pneumonia, or, in the most severe cases, death, with lungsshowing only the hemorrhagic and edematous type of lesion. The X-ray evidence,then, shows that some degree of pulmonary involvement is present in practicallyall cases of influenza, evidenced by a hazy opacity generalized throughout thelung, and a thickening of the bronchial structures and increased density oftheir shadows. This finding corresponds well with the generalized hemorrhagicand edematous lesion of the bronchi and parenchyma described on post-mortemexamination for cases dying early, and justifies the belief that substantiallythis condition in varying degree exists in milder cases that recover withoutcomplication.

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With the advent of definite secondary infection the platesshow a great variety of pictures. Briefly, the bronchopneumonic consolidationstend 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 tosuggest the "pussy willow." Still greater extension results inconfluence and the production of massive shadows resembling those of lobarpneumonia. When resolution occurs it progresses from the periphery toward thehilus, in inverse order of occurrence. Lobar consolidation shows almostsimultaneously throughout the area involved, with a tendency to peripheralinvolvement first and resolution is first central, the peripheral portionsretaining the shadow to the last. Interlobar pleuritis is frequentlydemonstrated by the X ray when no physical signs are to be discovered. It showsas a fine almost hairlike line in the anatomic location of the fissure involved.Pleuritic fluid and empyema give the well-recognized pictures of theseconditions.

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

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

CERTAIN CLINICAL ASPECTS

In the consideration of the clinical aspects of the respiratorygroup of diseases dealt with herein, only such features are touched upon asrepresent advances in knowledge during the World War, or as are of especialimportance in the recognition or treatment of the conditions described.

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

The classical descriptions of influenza divide the cases intovarious types, respiratory, gastrointestinal, meningeal, septicemic, etc. Thecases as they presented themselves during the cycle of 1918 were remarkable fortheir uniformity of type. Minor differences in symtomatology are noted in thedescriptions published from different sources but there is general agreementthat the respiratory type of the disease predominated greatly. It would appearthat, for this pandemic at least, influenza appeared as primarily involving therespira-

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tory tract and that symptoms referable to other organs werenot only rare, but seemed due either to toxemia or to extension of secondaryinfection. Whereever a definite outbreak occurred the cases were so strikinglysimilar in their clinical manifestations as to leave no doubt as to theiressential unity. During the severe outbreak there was a gradual change in thecourse of the cases, later cases running a more prolonged course with a greatertendency to the development of leucocytosis, and of secondary suppurations.

Though the clinical descriptions were in general very closelyfollowed in the uncomplicated cases, some unusual complications were relativelyfrequent. These latter are considered below. One point that appears important indiagnosis is the very general agreement that the throat presented acharacteristic appearance at onset. The various descriptions agree closely thatthe characteristic influenza throat showed a brilliantly red, glazed appearanceof the pharynx and fauces without swelling or exudation. This appearance wasmost marked on the soft palate, and the sharp delimitation of the reddening atthe margin of the hard palate was stressed. This throat condition was describednot only in connection with the spring outbreak but with that of the fall aswell. All patients complained of some degree of sore throat.

During severe outbreaks, many patients exhibited ahemorrhagic tendency. The frequency of this condition appears to have variedsomewhat with locality. It was more common in the Army in the United States thanin the American Expeditionary Forces. It was most commonly manifested byepistaxis, often recurrent, but hemmorhages from other mucous surfaces were notrare. Early and profound cyanosis of a peculiar color, described by some as"heliotrope," occurred in the more severe infections, especially infulminant cases and in those that later developed pneumonia. Extremeprostration, out of proportion to the degree of fever or the duration of theillness, was generally recognized. So, too, was the slow return to normal inconvalescence.

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

It has been recognized for years that the white blood countin influenza showed no hyperleucocytosis. This was confirmed during the pandemicunder consideration. Extremely low counts were found at the time of greatestvirulence of the infection. In the fall of 1918, almost any large series ofcounts showed a few as low as 2,000 or even lower. There is some evidencetending to indicate that counts lower than usual were of relatively badprognostic significance. The following figures from Camp Custer, Mich., indicatethat cases later developing pneumonia showed low counts on admission in greaterproportion 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. Thiscoincides with the usual experience. Of all cases at Camp Jackson, S. C., 42.2per cent had counts of less than 5,000, with an average of 6,344.⁷²This included complicated as well as uncomplicated cases. The average of casesthat developed pneumonia was 7,141. A report from Camp Hancock, Ga., at the timeof 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 arealso concordant in the different reports. There is general agreement that thepolynuclear elements are relatively reduced and the percentage of lymphocytesincreased. The lymphocyte percentage during the spring epidemic often exceeded50. In the later days of the outbreak this disproportion was not so marked, butstill remained evident. Experience in the severe fall outbreak did not show sohigh a percentage of lymphocytes. Thus from Camp Jackson, S. C., it was reportedthat there was an average of 35.5 in uncomplicated cases, 27.8 in those thatdeveloped pneumonia.⁷²The opinionwas generally expressed that there was no relative increase of polynuclearelements unless secondary infection impended. In general the statement is almostuniversal that the differential count was normal or showed a relativelymphocytosis.

The clinical types of pneumonia occurring during the 1918cycle of influenza differed so decidedly from those usually seen ininterepidemic periods and even from those described for previous pandemics thatsome description of these cases seems necessary here. A general view of thepneumonia situation during the World War shows that there was a gradual changein the predominating clinical type of the disease from the early cases, whichwere in no way noticeable as different from the usual type of pneumonia, to thevery atypical pneumonias of September and October, followed by a distincttendency to revert to the types of earlier months as the epidemic declined. Thischange was manifested in the relative proportions of lobar and ofbronchopneumonia, in changes in the proportion of cases developing secondarysuppurations, notably empyema.

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

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Pneumonia complicating measles was not of unusually frequentoccurrence. Particularly was this true as regards the earlier weeks of the greatmeasles epidemic in the camps. However, by December, 1917, when there was asudden increase in incidence of mild respiratory infections, variously recordedas influenza, bronchitis, coryza, etc., the number of cases of pneumoniaapparently primary, as well as cases associated with measles, rose suddenly toalarming proportions especially in some of the southern camps. The pneumonia ofthis period was carefully studied by a special commission of medical officersworking at Camp Travis, Tex.²Theynoted the presence at the time of an "epidemic of coryza, laryngitis, andmild bronchitis" in both the civil and military population of San Antonio.The report of the commission states that clinically the pneumonias studied couldbe divided into three groups: Bronchopneumonia, associated as a rule but notinvariably with a recent measles attack; (2) lobar pneumonia, giving thefamiliar signs and symptoms of that disease and sometimes following measles; (3)a group showing clinically and anatomically a combination of groups 1 and 2. Thedescription of the bronchopneumonia found there is of especial interest not onlybecause it was the unusual feature of the outbreak, but because it is of valuein comparing it with the bronchopneumonia of later waves. The cases at SanAntonio were nearly all associated with the hemolytic streptococcus, thoughnearly half of them showed the presence of the Pfeiffer bacillus as well. Onsetwas gradual and without definite chill or sudden elevation of temperature,whether occurring during the course of measles or only after an interval ofseveral weeks. The temperature rarely exceeded 104? F., and was frequentlyirregular even in the absence of empyema. The pulse rate was not extremely rapideven in cases near death. Respiration, too, was not extremely rapid but wascharacterized by extreme respiratory difficulty. Cyanosis was constant even inearly cases.^a The cough was troublesome and the sputum varied incharacter, though not showing the tenacious rusty type, typical of lobarpneumonia. Pain was usually marked and was associated with the frequency ofpleural infection. When noted, consolidation was usually at the base. R?les,musical, squeaking, or moist, were usually heard throughout the chest. In somecases characteristic signs of consolidation in a certain area persisted for afew days and then entirely disappeared. In uncomplicated bronchopneumonia, wideareas of dullness and tubular breathing were never observed. When such signswere found there was invariably a concurrent lobar pneumonia. Empyemacomplicated about one-half of the cases studied and its fatality was at least 50per cent. The lobar pneumonia studied showed the presence of pneumococci, theepidemic types being demonstrated in two-thirds of the cases. In the seriesshowing combined lesions both pneumococci and streptococci usually weredemonstrated.

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 timethe camps were much more generally affected. Empyema was still common and themortality was generally associated with this complication. A small number ofcases of a new type of the disease was seen for the first time during thisoutbreak. A patient with an attack of typical influenza of two or three days'duration would, after a day or two of normal temperature, develop acutepneumonic symptoms and die within 48 hours. This was the fulminant type ofinfluenzal pneumonia familiar in the fall outbreak. The proportion of casesrecorded as lobar in type was lower in the spring than in the winter and thecase fatality of pneumonia was higher.^78,79

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

The percentage of influenza cases developing pneumonia atthis time varied in different localities. The maximum figures were about 25 percent. The usual case fatality at this time was around 30 per cent. Though thedisease, as seen in different camps, varied somewhat in its clinicalmanifestations due to the various factors that have been discussed, the generalcharacteristics of the complication were very constant. Few observers were ableto distinguish clinically with any definiteness in the early stages of thedisease between cases which showed later lobar lesions and cases ofbronchopneumonia. The following condensed description of influenzal pneumonia ofthe 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 aftertwo or three days of normal temperature following an attack of influenza, or itdeveloped gradually without there being an afebrile interval. In the formergroup 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 characteristicallyslow; the blood pressure low, the systolic figure often below 100 mm.;respiration was only slightly accelerated. Nonfatal cases usually recoveredafter 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 moremarked, even to an indigo blue color, the temperature ranged somewhat higherthan in the unilateral cases and often showed variations paralleling the advanceand recession of the pulmonary lesion as shown by the X ray or by physicalsigns. Cough was frequent and exhausting; the sputum, blood tinged ormucopurulent. In the more toxic cases, terminating fatally, the color of thepatient from the first was either that of an intense cyanosis or a muddy,claylike pallor. The pallor was of particularly bad prognostic import. Nervoussymptoms appeared early, rest-

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lessness and delirium being marked. The respiration becamevery rapid and dyspnea was pronounced. Physical signs of irregular consolidationand of edema filled the entire chest. The temperature ranged to 105? F. orhigher, and death occurred in from three days to a week. It is evident thatthese groups were not clean-cut and that all degrees of varying severityintervened. Inasmuch as such a proportion of severe pneumonia has in the pastseldom been associated with influenza, it is important to record in somewhatgreater detail the peculiarities of this outbreak.

The first point to strike the observer was the universaloccurrence of cyanosis. This condition appearing in an apparently uncomplicatedcase of influenza, if of a degree at all marked, usually presaged the onset ofpulmonary inflammation. Whether due to toxic changes in the composition of theblood or to mechanical interference with oxygenation by the exudate in thelungs, the intensity of the cyanosis was, in general, an index to the severityof the case. In milder cases of influenza, a peculiar shade of "pinkcyanosis" was observed, an erythematous flush of an unusual shade. Thewell-established case of pneumonia showed a shade that was usually described asheliotrope, 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 themucous membranes was very notable. Epistaxis, which occurred in 10 per cent ormore of the cases, was of all degrees, but often severe, recurrent, anddebilitating in the extreme. Purpura, intestinal, and renal hemorrhages alsooccurred.

Of respiratory symptoms proper it may be said that thesediffered relatively little from the respiratory symptoms of the usualpneumonias. Pleuritic pain was frequent, cough was distressing, and frequentlythere was so much expectoration as to make resorting to narcotic relief seemdangerous. The character of the sputum varied from the tenacious rustyexpectoration of typical lobar pneumonia, through varying degrees of mucopus,and frothy blood-stained material to the profuse pink froth in the mouth andnose 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. Typicalsigns of consolidation were seldom found, and then late. After some experiencewith these cases most observers concluded that the diagnosis of pulmonaryinvolvement was better made from the general course and symptomatology than fromphysical signs. Here, too, the X-ray examination proved very valuable, as wasstated above. The early signs of pneumonia were confined to the presence of finescattered r?les, and as these r?les were found in many apparentlyuncomplicated influenza cases their significance was not clear. As theinvolvement proceeded, dullness became evident on percussion, and breath sounds,voice, and fremitus were diminished, thus suggesting fluid in the pleura. Areasof tympany were also observed. After several days the confluence, or extensionof consolidated areas, often produced typical signs of consolidation. Pleuriticfriction 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. Lowblood

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pressure was noted, in some cases the systolic blood pressurefalling as low as 80 mm. without a necessarily fatal issue. The temperature wasvery variable, usually of a fairly continous type, but in some cases remissionswith sweating were frequent even without suppurative complications. Theleucocyte counts were also variable, some fatal cases showing no change from theinitial leucopenia. In others a marked polynucleosis supervened. Pneumococcuscases showed this rise earlier than did cases infected with streptococci. Bloodcultures were positive in a relatively small proportion of cases, andpneumococcus infection gave the great majority of the positive results.

Toxic nephritis, varying in degree, occurred in nearly everycase. The presence of large numbers of casts was almost invariably of fatalimport. Gastrointestinal symptoms were rare, though early and persistentvomiting occurred in the highly toxic cases. Constipation was the rule. Toxicinvolvement of the nervous system was evident in all the more severe cases.There was sleeplessness, restlessness, severe headache and, to a greater orlesser degree, delirium. The delirium appeared to be related to the degree oftoxemia rather than to the temperature. Two types of delirium were noted: Arestless talkative type, hard to control, but unassociated with a lack oforientation when the patient was questioned; the coma vigil type. The talkativetype was not of very bad prognostic import, but almost all cases who had thecoma vigil type of delirium died.

Skin eruptions were prominent in some series of cases andhardly mentioned in others. At the Walter Reed General Hospital a reddisheruption of a maculopapular character, occurring typically on the chest andback, was seen in about two-thirds of the cases. It differed from acne in theabsence of pustules, and from sudamina in the absence of vesicles. It persistedinto convalescence and was followed by scaling. Profuse sweats occurred in thehighly toxic and in convalescents.

The case fatality of the pneumonias of this outbreak variedfrom 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 oftroops showed variation in this respect, thus suggesting that differentstandards were adopted in the diagnosis of pneumonia. From the account ofsymptoms and physical signs given above it is easily seen how this could havehappened. During the fall wave the greatest percentage of total strength dyingas 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 casesadmitted with a primary diagnosis of influenza from our troops in the UnitedStates 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 workoccasioned by the epidemic, many minor complications were not recorded, theabove figures probably represent with some accuracy the incidence ofcomplications and sequel? important enough to have an effect on the clinicalcourse of the case. It is seen that, with the exception of the respiratorycomplications, the number recorded under any heading is very small in proportionto the total number of admissions.

A number of reports on the incidence of otolaryngologicalcomplications have been published. Reports of the symptomatology of the diseaseindicate that catarrhal otitis media without perforation was of frequentoccurrence, though exact figures are not available. The figures in the abovetabulation probably represent fairly accurately those cases of otitis thatrequired special attention or operation. It is seen in the tabulation that thiscomplication occurred in 3,431 cases out of 734,397, or only 4.68 instances per1,000 influenza admissions. The rate of secondary otitis media for all medicalcases, exclusive of influenza, is 5.01 per 1,000. Measles had a rate of 41.9 per1,000, scarlet fever 35.2, and epidemic meningitis 21.6. There is no doubt thatthere is some slight tendency toward lowered resistance in parts other than thelungs during the course of influenza, but were the statistics available it iscertain that it would appear that the vast majority of these infections wereassociated with influenzal pneumonias of corresponding bacterial origin ratherthan with the primary disease itself.

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In general, complications were less frequent proportionatelyduring the height of the fall outbreak than was the case during the earlierwaves and during the period of decline after the fall wave. This is particularlytrue of the incidence of empyema.^a Several complications, while notnumerically important, are of great interest on account of the fact that theyappear to have attracted particular attention for the first time during thispandemic.

The first of these is subcutaneous emphysema.⁸⁷In this condition the subcutaneous areolar tissue becomes infiltrated withair over a greater or lesser extent of the body. It usually began above theclavicles or manubrium and extended in some instances until practically theentire body was affected. Its occurrence was extremely irregular, some largeseries of cases being reported without noting it and others reporting severalcases 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 inadjoining beds. This distribution led to the consideration of an infectiveorigin for the complication and from 4 of the cases an anaerobic spore-bearinggas former was isolated. However, the great majority of cases failed in otherhands to show any such origin, and the generally accepted theory of thepathogenesis of the emphysema, based on careful autopsy studies, is that it isthe result of rupture of the dilated bronchioles, the air passing along thevessel sheaths to the mediastinum and thence to the subcutaneous tissue. Theslow dissection thus accomplished by the air is remarkably painless; and whileits occurrence is prognostically bad, by no means all of the extensive caseswere fatal.

Another interesting complication is the degeneration of therectus muscle, usually accompanied by rupture and hemorrhage. After attentionwas called to this occurrence a few instances were reported in almost everyautopsy series. McCallum⁷⁹noted itin the 1918 spring epidemic. The primary lesion appears to be a hyalinedegeneration of the muscle fibers with loss of striation, similar to if notidentical with the condition known as Zenker's degeneration. When rupture andhemorrhage are added, bacterial invasion of the area may result in the formationof abscess. This condition doubtless accounts for many of the instances ofabdominal pain and rigidity, simulating peritonitis, that were observed duringthe epidemic.

Inflammation of the accessory sinuses of the nose, whilerarely giving rise to clinical symptoms, was almost invariably foundpost-mortem. The postorbital headache of the early days of the disease has beenattributed to sphenoidal sinusitis. There has been a fairly prevalent beliefthat influenza was frequently followed by pulmonary tuberculosis. It is seenfrom the tabulation given above that in 956 instances the diagnosis oftuberculosis, secondary to influenza, was recorded. This amounts to 1.3instances per 1,000 admissions. In all noninfluenzal admissions this diagnosiswas recorded secondarily in 1.5 of every 1,000. These figures should perhaps beaccepted with caution owing to the fact that the great majority of the menaffected by influenza were discharged from the service within a few months timeand late-developing tuberculosis might have been missed. However, the mortalitystatistics of the registration area for the years following 1918 have shown aprogressive 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 veryimprobable that any great number of cases of tuberculosis owe their origin tothe influenza epidemic. The same general conditions hold for neurocirculatoryasthenia. 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 ofinfluenza, no specific remedies are available for its cure. The currentconception of this disease as a relatively mild respiratory infection, short induration and leading to fatal results only when complicated by secondaryinfections, usually pulmonary, results in a treatment logically directed toshorten its course, to limit the amount or degree of primary pulmonary damage,to protect the patient against secondary infection from his fellows, and toreduce if possible by these means the proportion of fatal pulmonarycomplications. Further treatment aiming to promote the comfort of the patient isthe second line of attack. When pneumonia has developed there are severalmethods of treatment in influenzal cases that do not apply in interepidemicperiods, but in general the disease is best treated along orthodox lines.

INFLUENZA

Since the main aim of treatment in the uncomplicated earlyinfluenza case is the avoidance of pulmonary complications, the results oftreatment are best estimated by consideration of the percentage of recoverieswithout pneumonia. The principles of treatment best adapted to this end havebeen well established, although definite statistical evidence of the same cannot be given here owing to the fact that other factors predominated indetermining the severity of cases as occurring in different localities. Thesefactors have been included in the consideration of the epidemiology and of theprevention of the disease (vide supra).

Experience, however, led to the crystallization of thegeneral opinion that certain measures resulted in reducing the proportion ofpneumonia cases. Of these the first and perhaps most important was the earlyinstitution of treatment. Men who continued on duty after definite symptoms haddeveloped were much more likely to develop pneumonia. The excellent morale ofthe combatant troops in the face of the enemy, which led many soldiers to refuseto report themselves sick until forced to do so, is believed to be one greatcause of the greater proportion of pneumonias and relatively high fatality shownby the troops in the American Expeditionary Forces.⁸⁸The important elements of treatment, once the patient comes under medicalcare, were found to be rest in bed, warmth, and a light, hot diet. It is theconsensus of opinion that under such treatment the great majority of cases areconvalescent within two to three days. The question of open-air treatment hasbeen much debated, but the weight of opinion is to the effect that open-airtreatment is only permissible when it may be maintained without sacrificing thewarmth of the patient. Drug treatment is of a palliative character. Aspirin waslargely used for the pains of onset, though it was criticized by some as beingdepressant. Dover's powder, or morphine, to promote rest; sprays, preferablyoily, to relieve naso-

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

PNEUMONIA

The general principles applicable to the uncomplicated influenzacases in regard to rest, warmth, and ventilation apply equally here. The usualdrug medication was generally used without striking success. Specific treatmentwith antipneumococcus serum in type I cases was generally used and showedgenerally satisfactory results.⁸⁹Otherattempts at specific treatment directed against the pneumococcus included theuse of polyvalent antipneumococcus sera, the use of the Kye's antipneumococcuschicken serum, the autolyzed pneumococcus antigen of Rosenow, and thetherapeutic use of bacterial vaccines.⁹⁰Favorable 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 ofconvalescents. It is to be noted that all except the last of these measuresinvolves the introduction into the circulation, usually intravenously, ofprotein products foreign to the human system. This is also true of the type Iserum, the effects of which, however, are so much more clean-cut than those ofthe others that its specific action may hardly be questioned. Theseconsiderations have led many to the belief that a nonspecific protein reactionis 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 thatnot a few cases reported as other than type I showed benefit from the serumtreatment. It was also the prevalent belief that in cases in which a chillfollows the administration there was increased likelihood that 12 hours laterthe temperature would be much lower and the general condition improved.

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

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

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fatality there was appreciably lower than in neighboringcamps under apparently the same climatic conditions and made up of the same typeof men. The solution, from 5 to 25 per cent in strength, was given in amounts of250 c. c. intravenously once or twice daily.

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

In general it may be said that the experience of the WorldWar has confirmed the position of the antipneumococcus serum, type I, when givenin sufficiently high titre and proper dosage; it has led to a widespread beliefin the beneficial effect of nonspecific protein reactions however induced and anequal belief in their essential harmlessness; it has shown the beneficial effectof the intravenous use of hypertonic glucose solutions, although experience withthis agent was not general; and, lastly, owing to the universal agreement of themany who made use of convalescent serum in some form as to the good effect ofthis treatment, it appears established that the serum of convalescents containscurative antibodies.⁹⁶This lastobservation encourages the hope that with the discovery or recognition of theprimary etiological agent of influenza a hyperimmune serum may be developed inanimals which may at least prove effective in limiting the amount of pulmonarydamage done by the primary disease and thus prevent the pneumonic complications.

REFERENCES

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

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

(3) Report of the pneumonia commission at CampWheeler, 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 bythe Department of Commerce, Bureau of the Census.

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

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

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

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

(10) Hall, Milton W., Maj., M. C.: A PossibleFallacy 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 campsurgeon, to the Surgeon General. On file, Record Room, S. G. O., 710-1 (name ofcamp) 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 SurgeonGeneral, 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 UnitedStates During the Six Months from September 29, 1917, to March 29, 1918. TheJournal 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, madeby 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 inthe Distribution of Influenza Virus, and the Secondary Invaders. The MilitarySurgeon, 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 ofAcute Respiratory Infection at Camp Custer, Michigan. The Journal of theAmerican Medical Association, 1918, lxxi, No. 24, 1918.

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

(28) Jackson, Thomas W.: The Other Side of the Question ofIndirect 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 ofRespiratory Diseases in the Army. The Journal of Laboratory and ClinicalMedicine, 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 InfluenzaExperimentally. 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 ofMixed Vaccines against Pandemic Influenza and its Complications. The Journal ofthe 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. Onfile, Record Room, S. G. O., 710-1 (Camp Lee) D.

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

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

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

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

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

(41) A summary of the epidemiological evidence as to therelation between typhoid inoculation and the incidence of pneumonia, made byMaj. 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 inincreasing Susceptibility to Other Diseases. The Journal of the American MedicalAssociation, Chicago, 1918, lxxi, No. 26, 2146.

(43) The relation of typhoid vaccination to the incidence ofpneumonia. 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 StreptococcusPneumonia and Empyema at Camp Dodge, Iowa. The Journal of the American MedicalAssociation, 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 ProphylacticInoculations against Pneumococcus in 12,519 men. The Journal of ExperimentalMedicine, 1918, xxviii, 19.

(49) Cecil, R. L., and Vaughan, H. F.: Results of ProphylacticVaccination against Pneumonia at Camp Wheeler. The Journal of ExperimentalMedicine, 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, RecordRoom, 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. ThePractitioner, 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 anEpidemic of Influenza in an Army Post of the American Expeditionary Forces inFrance. On file, Record Room, S. G. O., 710-1 (A. E. F.; France) Y.

(59) Special report from Camp Shelby, Miss. On file, RecordRoom, 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, RecordRoom, S. G. O., 710 D.

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

(62) Special report from Camp Cody, N. Mex. On file, RecordRoom, 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 CommunicableDiseases at Replacement. Depots. The Military Surgeon, Washington, 1919, xlv,No. 1, 59.

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(65) Cornwall, E. E.: Spanish Influenza; Cases of Influenzaand 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.; andWheeler, G. W.: The Epidemic of Influenza at Camp Sherman, Ohio. The Journal ofthe American Medical Association, Chicago, 1918, lxxi, No. 20, 1652.

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

(68) Report from the camp surgeon, Camp Grant, Ill., relativeto 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 (CampDevens) D.

(70) Report from Camp Travis, Tex., relative to influenzaepidemic. 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 AurensPneumonia. The Journal of the American Medical Association, Chicago, 1919,lxxii, No. 9, 617.

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

(74) MacCallum, W. G.: Pathology of the Epidemic StreptococcalBroncho-pneumonia in Army Camps. Monograph of the Rockefeller Institute forMedical Research, 1919, No. 10; also The Journal of the American MedicalAssociation, 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 ofthe American Medical Association, Chicago, lxxii, No. 8, 556.

(76) Winternitz, M. C., Wason, I. M., and McNamara, F. P.: ThePathology 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 AmericanMedical 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 StreptococcalBronchopneumonia, in Army Camps. The Journal of the American MedicalAssociation, Chicago, 1918, lxxi, No. 9, 704.

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

(81) Wolbach, S. B.: Comments on the Pathology andBacteriology of Fatal Influenza Cases As Observed at Camp Devens, Mass. JohnsHopkins 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 ReedGeneral 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 Useof 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 theAmerican Medical Association, Chicago, 1919. lxxii, No. 21, 1520.

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

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

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(87) Subcutaneous emphysema complicatingbroncho-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 thefascial 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 andspontaneous 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 ArgonneInfluenza Epidemic. New York Medical Journal, New York, 1919, cx, No. 23, 925.

(89) Spooner, L. H., Sellards, A. W., andWyman, J. H.: Serum Treatment of Type I Pneumonia. The Journal of the AmericanMedical Association, Chicago, 1918, lxxi, No. 16, 1310. Also, A group of 68cases of Type I pneumonia occurring in 30 days at Camp Upton, with specialreference 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 thetreatment 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 oflobar 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 23and 30, 1919, cx, 316, 368.

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

(93) Snyder, R. G.: Spanish Influenza; ItsTreatment by the Use of Intravenous Injections of a Non-Bacterial Split Protein.New York Medical Journal, New York, 1918, xviii, 843. Also, The use ofintravenous injections of hypertonic glucose solutions in the treatment ofinfluenzal 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 adjunctmeasure in the therapy of pneumonia, Capt. Henry J. John, M. C. On file, RecordRoom, S. G. O., 710-1 (Ft. Sam Houston) N.

(94) Report of the pathology of influenza andpneumonia, 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. TheJournal 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. TheJournal of the American Medical Association, Chicago, 1918, lxxi, No. 24, 1992.Also: Ross, C. W., and Hund, E. J.: Transfusion in the Desperate PneumoniasComplicating Influenza. Journal of the American Medical Association, Chicago,1918, lxxi, No. 24, 1992. Also: Kahn, M. H.: Serum Treatment of PostinfluenzalBroncho-pneumonia. Journal of the American Medical Association, Chicago, 1919,lxxii, No. 2, 102. Also: Maclachlan, W. W. G., and Fetter, W. J.: Citrated Bloodin the Treatment of the Pneumonia Following Influenza. Journal of the AmericanMedical Association, Chicago, 1918, lxxi, No. 25, 2053. Also: Browne, W. L., andSweet, B. L.: Treatment of Influenzal Pneumonias by Citrated Transfusions.Journal of the American Medical Association, 1918, lxxi, No. 19, 1602.