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

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

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

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

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

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

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

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

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

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

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

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

ANNUAL ADMISSION RATES PER 1000 STRENGTH

10

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

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

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

with a varying proportion of pneumonia. The summer of 1918 was one of troop movements on an enormous scale. Camp after camp sent its trained division abroad, and was filled up with recruits. Before leaving, many divisions received recruit detachments to complete their strength. At Camp Funston, Kans., several such drafts were received during the summer, each one within a few days of its arrival in camp being attacked by influenza and pneumonia.11 These recurring epidemics in recruits were carefully studied by a special board and a full report was published. At Camp Cody, N. Mex., in June, the incoming draft suffered from "bronchitis" and pneumonia.16 The same sequence was observed elsewhere to a greater or lesser extent. It has been shown above that the rates for influenza and pneumonia began to rise early in August, and increased progressively from that time to the high point in the fall. It is noticeable that the mortality rate of these cases increased through the warm months of the year, when mortality from respiratory disease is at its minimum.

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

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