The bacteriologic investigation which will be described was made at Camp Pike, Arkansas, during the period of the influenza epidemic from September 6 to December 5, 1918. The data presented are limited to observations made during life in uncomplicated cases of influenza and to control studies in normal individuals, and in cases of measles. Bacteriologic studies made at autopsy will be described in a subsequent part of this report.

Because of the wide variations in opinion concerning the relationship of various bacteria to influenza that have arisen during the progress of the recent pandemic, a brief review of the salient features of the earlier literature seems advisable. In 1892 Pfeiffer^[8] found a small, Gram-negative, hemophilic bacillus in all cases of influenza, often in almost pure culture, both during life and at autopsy. He stated that the organism was found only in cases of influenza or in those convalescent from the disease. Similar bacilli occasionally found in other conditions he classified as pseudoinfluenza bacilli. He furthermore showed that freshly isolated cultures were pathogenic for monkeys, producing a disease not unlike influenza, though lacking in what he considered the characteristic lung lesions. He therefore felt justified in claiming that this bacillus, which he designated B. influenzÆ, was the cause of epidemic influenza. Pfeiffer’s work, though hailed by many as unassailable, has failed to stand the test of time in two respects. It has been definitely shown, by Wollstein^[9] in particular, that there is no justification for recognizing a group of pseudoinfluenza bacilli, organisms so classified by Pfeiffer being indistinguishable from B. influenzÆ. Furthermore, numerous investigations have demonstrated that B. influenzÆ may frequently be found in a variety of diseases affecting the respiratory tract and in a small proportion of normal individuals. Kretz^[10] found it 47 times in 950 examinations, usually associated with disease of the respiratory tract. SÜsswein,^[11] Liebscher,^[12] Jehle,^[13] Wollstein,^[9] Davis^[14] and many others have demonstrated its presence in cases of measles. Lord^[15] isolated B. influenzÆ in 30 per cent of 186 sputums from patients with acute and chronic infection of the respiratory tract. Boggs^[16] found it in frequent association with chronic bronchiectasis. Wollstein^[9][17] showed that it was often present in the respiratory diseases of infants, and was not an infrequent cause of meningitis. Rosenthal^[18] found that one in six of normal individuals harbors influenza bacilli and therefore considered it purely a saprophyte, a position, of course, thoroughly untenable in the face of indisputable evidence that it may be highly pathogenic. The widely accepted statement that B. influenzÆ is nonpathogenic for animals has apparently served in considerable degree to shake belief in its etiologic relationship to epidemic influenza. It would appear, however, that this opinion is not founded upon fact. Reference is again made to the work of Wollstein^[19], who has shown that virulent strains of B. influenzÆ, when freshly isolated from the human host, are highly pathogenic for rabbits and monkeys and that nearly all strains are more or less pathogenic for mice and guinea-pigs.

None of these modifications of Pfeiffer’s original work, however, would seem to constitute any valid reason for abandoning the conception of the etiologic importance of B. influenzÆ. On the contrary, they are quite in harmony with well-established facts concerning other bacteria which cause infections of the respiratory tract. Such bacteria are frequently found in normal individuals leading a saprophytic existence, are often associated with other disease conditions, and tend to show marked variations in virulence.

Since the outbreak of scattered epidemics of influenza beginning in 1915–16, which finally culminated in the pandemic of 1918–19, a vast amount of literature on the subject has appeared. No attempt has been made thoroughly to analyze this, because much of it is not available, much of it abounds in contradictions which it is difficult to harmonize at the present time, and much of it has been written on the basis of insufficient data gathered under the handicap of war conditions by men without sufficient time to undertake special investigation, or it is feared, in many instances, not sufficiently qualified by previous bacteriologic training.

The sum and substance of opinion in 1918 would seem to be best summarized by quoting from the published report compiled by the British Medical Research Commission:^[20] “Although Pfeiffer may yet furnish reasons why the verdict should not be pronounced, there is already sufficient material to shake the orthodox conception out of its high altar. Two facts stand out prominently: the generally acknowledged, or by some reluctantly admitted, absence of B. influenzÆ from organs on postmortem examinations, and the universally recorded findings of diplostreptococci, singly or in association with the Pfeiffer bacillus.” Comment on this opinion will be made in the general discussion at the end of this paper.

In undertaking a study of the bacteriology of influenza, it seemed essential to bear in mind certain clinical features of the disease which will be discussed in greater detail in a subsequent paper. It suffices to say for our present purpose that it is felt that influenza in itself should be regarded as a self-limited disease of short duration (two to five days in most instances), the most prominent local manifestation of which is a rapidly progressing attack upon the mucous membranes of the respiratory tract. Among the cases observed during the epidemic at Camp Pike uncomplicated influenza never proved fatal and death invariably was associated with a complicating pneumonia. In a large majority of cases pneumococci, S. hemolyticus, or less frequently other bacteria in addition to B. influenzÆ were associated with the pneumonia. It is felt, therefore, that in any attempt to determine the primary cause of influenza bacteriologic studies made during life in early uncomplicated cases of the disease are of primary importance and that the bacteriology of the sputum of patients with complicating pneumonia and the bacteriology of autopsies can only properly be used as valuable supplements to data so obtained.

Since cultures from the respiratory tract must often of necessity contain many bacteria which play no part in the production of influenza, it is essential to have a working knowledge of the bacteria that may be encountered by the methods employed. It is also important that such knowledge as may have been gained in interepidemic periods be amplified by study of the bacterial flora present at various periods throughout the course of an epidemic, both in normal individuals and in other disease conditions. These points have been borne in mind throughout the present study and such observations have formed an essential part of the work.

Methods.—In an investigation of this nature the culture methods employed should be suitably directed to determine primarily what bacteria are present and in what relative proportion they exist. The use of culture or animal inoculation methods that are highly selective in character, enhancing the growth of certain bacteria and retarding or inhibiting the growth of others, are of great additional value, but can only properly be used secondarily in order to augment the results obtained by nonselective culture methods. As the most suitable medium for the purpose in hand plain meat infusion agar, titrating 0.1+ to 0.3+ to phenolphthalein, to which 5 per cent of sterile defibrinated horse blood was added, was used. Since growth on freshly poured plates is greatly superior to that on plates that have been stored, the agar was melted as needed, the blood being added when the medium had cooled to approximately 45° C. Cultures from the nose and throat were made by swabbing the mucous membranes with a sterile applicator, touching the applicator to a small area on the surface of a blood agar plate, and spreading the inoculum over the surface of the medium with a platinum needle, insuring as wide a separation as possible. Direct cultures of selected and washed specimens of sputum were made when possible. In many instances, of course, it was impossible to get sufficiently satisfactory specimens to permit of washing, especially when cultures were made very early in the disease. To supplement direct culture of the sputum the mouse inoculation method as employed for the determination of pneumococcus types was used. This is, of course, a highly selective method, of particular value in the detection of pneumococcus and B. influenzÆ when they are present in relatively small numbers as compared with other bacteria. Plates were examined after twenty to twenty-four hours’ incubation and again at the end of thirty-six to forty-eight hours when necessary.

In the present study, attention has been centered upon B. influenzÆ, S. hemolyticus, and the various immunologic types of pneumococci, other organisms encountered having played no significant part in the cases studied except in rare instances. B. influenzÆ was identified by its morphologic, staining and cultural characteristics and conformed to the classical description given by Pfeiffer. S. hemolyticus was identified by its morphologic, staining, and cultural characteristics on blood agar, supplemented by a confirmatory hemolytic test with washed sheep corpuscles, and bile solubility test. Pneumococci were identified by morphologic, staining and cultural characteristics, bile solubility test, and agglutination with specific antipneumococcus immune sera. Note was made in most instances of the presence of other organisms, such as members of the Gram-negative diplococcus, staphylococcus, diphtheroid and streptococcus viridans groups, but no attempt was made further to isolate or identify them.

Bacillus InfluenzÆ in Cases of Influenza.—On October 10, 1918, at the height of the epidemic at Camp Pike, search for B. influenzÆ was made in a group of 23 consecutive cases of uncomplicated influenza from one to six days after the onset of the disease. From each individual simultaneous cultures on blood agar plates were made (a) from the nose, (b) from the throat, and (c) from the sputum, and the sputum from each case was injected into the peritoneal cavity of a white mouse. A similar study of 5 consecutive cases was made on November 19. The results are presented in Table II.

By means of multiple cultures taken simultaneously from different portions of the respiratory tract no difficulty was encountered in demonstrating B. influenzÆ in all these cases of uncomplicated influenza. Not only was B. influenzÆ found in all cases, but often in very large numbers predominating over all other bacteria on at least one of the plates from each patient, and in occasional instances occurring in nearly pure culture. One culture made about two hours after onset of the initial coryza is of interest. There was at the time a profuse serous nasal discharge. One drop of this allowed to fall on the surface of a blood agar plate gave a practically pure culture of B. influenzÆ.

During the latter part of November and in early December a small secondary wave of influenza occurred at Camp Pike. In a series of 48 consecutive cases, B. influenzÆ was readily found in all by means of combined throat cultures and mouse inoculation of the sputum, 33 times (68.7 per cent) in the throat cultures, 39 times (81.3 per cent) in the sputum. These cases were cultured on admission to the receiving ward of the hospital within twenty-four to forty-eight hours after onset and were all early cases of influenza without complications at the time the cultures were made. In 90 more consecutive cases in this series 62 or 68.9 per cent showed B. influenzÆ in a single throat culture taken on admission.

A summary of all cultures made in cases of uncomplicated influenza is presented in Table III.

Of any single method used the intraperitoneal inoculation of a white mouse with a specimen of the patient’s sputum proved the most efficient in demonstrating the presence of B. influenzÆ. No single method served to demonstrate B. influenzÆ in all cases, but by simultaneous cultures from the nose, throat, and deeper air passages no difficulty was met in showing that B. influenzÆ was invariably present, usually in abundance somewhere in the respiratory tract during the acute stage of the disease. This result is not out of harmony with the rapidly progressive character of the attack upon the mucous membranes of the respiratory tract in influenza.

Of interest in this connection are certain observations which suggest that the presence of B. influenzÆ in predominant numbers at least is in many cases coincident with the acute stage of influenza and that the organisms show a tendency rapidly to diminish in abundance with the progress of the disease to recovery. In 82 cases of influenza cultured on the day of admission to the hospital, B. influenzÆ was present in 52 (63.4 per cent) of the throat cultures. Repeated throat cultures in this group of cases from the fourth to the eighth day after admission when the temperature had fallen to normal, showed that B. influenzÆ was still present in demonstrable numbers in the throat of only 25 cases or 30.5 per cent. Not only was there a material reduction in the number of patients in whom B. influenzÆ could be demonstrated by the throat culture method, but the contrast in the predominance of B. influenzÆ on the plates made early in the disease with those made during convalescence was often very striking. It is only fair to say, however, that some cases continued to carry B. influenzÆ in their throats in large numbers throughout the period of observation.

Presence of Pneumococcus in Cases of Influenza.—It seemed of some importance to determine the prevalence of pneumococcus in cases of influenza, not because of any possibility that pneumococci might bear an etiologic relationship to the disease, but more by way of comparison with the prevalence of B. influenzÆ, since both organisms are found in the mouths of normal individuals and are also frequently found together in the pneumonias that complicate influenza. The results obtained in cases of influenza early in the disease before the development of either a purulent bronchitis or of pneumonia are presented. The presence of pneumococcus was determined by the intraperitoneal inoculation of white mice with the saliva or sputum.

Twenty-four cases examined on September 27 and 28 gave the results shown in Table IV. These patients had been in the hospital from two to five days at the time the determinations were made.

From November 27 to December 1, the pneumococci present in 47 consecutive cases of influenza were determined. In this group specimens of sputum were collected shortly after admission of the patients to the receiving ward of the hospital. The results are shown in Table V.

The results obtained show that pneumococci found in early uncomplicated cases of influenza, both early and late in the course of the epidemic, differ in no respect from those found in the mouths of normal individuals at any time.

Similar studies of the prevalence of S. hemolyticus as determined by throat cultures in early cases of influenza are shown in Table VI.

The only point of interest in these observations is the increased prevalence of S. hemolyticus in cases examined late in the epidemic of influenza as compared with that found early in the epidemic. The significance of this will be discussed in other parts of this report.

Presence of Bacillus InfluenzÆ in Normal Men.—For comparison with the results obtained in cases of influenza a fairly extensive study of the prevalence of B. influenzÆ in normal individuals has been made at various times prior to and throughout the course of the epidemic. This was deemed of special importance, since it was obvious that the results obtained by previous workers during interepidemic periods would not in all probability coincide with those obtained in the presence of a widespread epidemic of influenza where the opportunity for the dissemination of B. influenzÆ was almost unlimited.

From the results obtained in the multiple cultures in cases of influenza it is obvious that only like methods can be compared. The results obtained in normal individuals have, therefore, been tabulated in groups dependent upon the culture method employed. These groups have been subdivided according to the time and the place of the study, such explanatory notes as seem necessary being added. (See Tables VII-IX.)

The most striking feature of the figures presented in Table VII is the wide variation in the incidence of B. influenzÆ in different groups varying all the way from 11.1 to 68 per cent. Analysis of these differences brings out certain points of great interest. It is apparent that the percentage of cases carrying B. influenzÆ depended in large part upon the prevalence of respiratory diseases in the group from which the data were obtained. In the studies made at Camp Funston prior to the fall outbreak of influenza in epidemic proportions, it is noteworthy that “bronchitis” and pneumonia were prevalent throughout the summer in those groups showing a relatively high incidence of B. influenzÆ. At the time these studies were made the presence of influenza in these organizations was not recognized, but in view of knowledge gained throughout the course of the epidemic at Camp Pike, it seems not improbable that influenza in mild form was present throughout the summer in certain organizations at Camp Funston. This would seem more likely in view of the fact that this commission has clearly demonstrated that a considerable epidemic of influenza swept through Camp Funston in March, 1918, and was followed by recurring smaller epidemics in April and May.^[21] In contrast with these groups showing a high incidence of B. influenzÆ is that of the 210th Engineers, an organization entirely free from respiratory diseases during the period of our study.

On November 12 search was made for B. influenzÆ in 50 normal drafted men who had assembled at Hot Springs, Ark., on that date preparatory to entraining for Camp Pike. These men were all from isolated farming communities where influenza was only moderately prevalent and where there was little opportunity for the wide dissemination of B. influenzÆ such as occurs when large bodies of men are assembled in camps. Twelve of the 50 gave a history of influenza within the preceding eight weeks. The cultures were made by the same methods as those used at Camp Pike, the laboratory car “Lister” being taken to Hot Springs for that purpose. The incidence of B. influenzÆ was only 22 per cent. In striking contrast with this figure are the figures of 50 and 68 per cent obtained in the last two groups studied at Camp Pike after the epidemic had swept through the camp: 24 of the 51 men in these groups had influenza during the epidemic.

It is of interest to record that the incidence of pneumococcus in these cases was approximately the same in all groups and bore no relation to the prevalence of influenza, bronchitis, or pneumonia.

The results obtained by throat culture are quite similar to those obtained by the mouse inoculation method. The entire absence of B. influenzÆ in the group of 64 throat cultures made in the draft men assembled at Hot Springs as compared with the relatively high incidence in the last two groups examined at Camp Pike is very striking.

In consideration of the figures presented in Table IX it is important to remember that the group of 50 men from Hot Springs were all from isolated farm communities, had not previously been assembled and had not been in continuous contact with a widespread epidemic of influenza. On the other hand, the two groups of normal men at Camp Pike were studied immediately after the epidemic had swept through the camp and had been constantly in contact with epidemic influenza for a period of three months, 24 of the 51 actually having had the disease during this period. The fact that in the group of men from Hot Springs, B. influenzÆ was found only by the mouse inoculation method is noteworthy, since it indicates that the organism was present in relatively small numbers and could be detected only by a highly selective method.

Summary of the results obtained in normal men shows that the incidence of B. influenzÆ in normal individuals from isolated communities or in groups free from respiratory diseases prior to the occurrence of the fall epidemic was relatively low, namely, 10 to 20 per cent; that in observations made before the fall epidemic in groups in which “bronchitis” and pneumonia were fairly prevalent, B. influenzÆ was found much more frequently, namely, in 25 to 50 per cent of the cases; and that in groups studied at intervals during the epidemic the incidence of B. influenzÆ rapidly rose, reaching 85 per cent at the end of the epidemic. In contrast with this, B. influenzÆ was found in 100 per cent of cases of influenza without reference to the time at which they occurred during the epidemic. It is obvious that the high percentage of normal men carrying B. influenzÆ found at the end of the epidemic can depend only on the wide dissemination of B. influenzÆ that must occur during epidemic times.

Bacillus InfluenzÆ in Measles.—Since the presence of B. influenzÆ in other diseases than influenza has been advanced as an argument against its causal relationship to influenza, an extensive study of the incidence of B. influenzÆ in the throats of measles patients was made during the period of the epidemic of influenza at Camp Pike from September 10 to October 20. In all a total of 830 throat cultures in 487 cases of measles were made, many cases being cultured repeatedly at weekly intervals. The results have been condensed as far as possible and are presented in Tables X, XI, XII.

The prevalence of B. influenzÆ in cases of measles during the period of the influenza epidemic corresponded very closely with that found in normal individuals under similar circumstances. The increasing proportion of cases carrying B. influenzÆ as the epidemic of influenza advanced is further evidence of the wide dissemination of the organism during the epidemic.

It is evident from the figures presented in Table XII that a large percentage of the measles cases studied were at one time or another carriers of B. influenzÆ. In consideration of this fact, it must be borne in mind that all these cases were cultured during the period when the influenza epidemic was at its height and that many of these cases had influenza while in the hospital for measles. No data are available as to the exact number, since a definite diagnosis of influenza could hardly be made during the acute stage of measles. It is probable that approximately 25 per cent developed influenza, since that was the incidence of influenza in the total population of Camp Pike. The consistent increase in the percentage of influenza carriers clearly demonstrates that this was due to wide dissemination of B. influenzÆ with the progress of the epidemic. Another point of exceeding interest is that the percentage of measles cases carrying B. influenzÆ in the throat was lowest during the acute stage of the disease and increased during convalescence. This is in direct contrast with the results found in cases of influenza where the number of cases carrying B. influenzÆ in the throat was highest during the acute stage and rapidly diminished in uncomplicated cases with the onset of convalescence.

Summary.—Multiple cultures made simultaneously from the nose, throat and lower respiratory tract showed that B. influenzÆ was invariably present in all cases of influenza from the onset of the disease. Not only was B. influenzÆ present in all cases, but it was frequently present in predominant numbers, sometimes in nearly pure culture. In the majority of cases that went on to rapid recovery without the development of an extensive bronchitis or complicating pneumonia, the predominance of B. influenzÆ over other organisms rapidly diminished coincident with onset of convalescence. Many cases, however, continued to carry B. influenzÆ in large numbers in the throat throughout convalescence. No data on the possible duration of the carrier state have been obtained. By the culture methods employed no other organism has been found that would suggest any etiologic relationship to the disease. The two organisms most frequently associated with B. influenzÆ in postinfluenzal pneumonias, pneumococcus and S. hemolyticus, have not differed in their incidence in early uncomplicated cases of influenza from that found in normal individuals.

The incidence of B. influenzÆ in normal men, in different groups studied, has varied between 11.1 and 88 per cent. This wide variation has depended upon the prevalence of respiratory diseases, more particularly influenza, in the groups studied and the opportunity thereby offered for the wide dissemination of B. influenza. With the progress of the epidemic, the number of normal men carrying B. influenzÆ has steadily increased until it reached its maximum at the end of the epidemic.

The incidence of B. influenzÆ in cases of measles studied during the epidemic of influenza has been relatively high though never equaling that found in cases of influenza. As in normal men, the incidence in cases of measles has steadily increased during the period of the epidemic. Repeated throat cultures at weekly intervals in cases of measles have shown that approximately 80 per cent became temporary carriers of B. influenzÆ at one time or another during the period of the epidemic. Many of these cases had influenza during the time that they were in the hospital. The carrier state in cases of measles was found to bear no relation to the acute stage of the disease since the number of carriers at the time of admission to the hospital was considerably lower than that found during convalescence as determined by repeated cultures in the same cases.

Discussion

The bacteriologic studies in cases of influenza described in this report fully support Pfeiffer’s claim that B. influenzÆ is invariably present in the disease. It is particularly important to note that these results were obtained in early uncomplicated cases of influenza and are not dependent upon cultures made from cases complicated by pneumonia or obtained at autopsy. In view of this fact the tendency so apparent in much of the recent literature to relegate B. influenzÆ to a place of secondary or minor importance in the disease seems hardly justifiable. It would seem that this tendency is largely dependent upon three factors: first, the failure of many to find B. influenzÆ either during life or at autopsy in any considerable proportion of cases; second, the frequent failure to draw a clear distinction between influenza itself and the pneumonia to which it predisposes with a consequent overemphasis upon autopsy bacteriology where a considerable variety of secondary organisms have attracted particular attention; and third, an incorrect interpretation of the undoubtedly large number of B. influenzÆ carriers found among normal individuals and those with other diseases during the period of the epidemic and to less extent in interepidemic times.

Since the majority of workers who are thoroughly familiar with the technic of cultivating B. influenzÆ have encountered little difficulty in finding it in a large majority of cases, it is felt that the considerable number of negative reports that have appeared can depend only upon the unfamiliarity of those who have failed to find it with the proper bacteriologic methods. This is quite apparent in many of the reports that have been published, and is not surprising in the face of the excessive demand for well-trained bacteriologists occasioned by the war.

One important feature in the successful isolation of B. influenzÆ from all cases that has been brought out in the course of the work here reported, is the necessity of making simultaneous cultures from all portions of the respiratory tract, since by no single culture method was it found possible to find the organism in all cases. It has been pointed out that one of the most characteristic local phenomena of the disease is the rapidly progressing attack upon the mucous membranes of the respiratory tract. It seems quite possible that B. influenzÆ in predominant numbers at least may be found in many cases only at the crest of the wave, if we may speak of it as such. By way of analogy is the well-recognized fact that the successful isolation of streptococcus from cases of erysipelas often depends upon taking cultures from the margin of the advancing lesion. While definite proof is lacking for this opinion, it would seem to receive some support from the observation that B. influenzÆ rapidly disappears from the throat with the onset of convalescence in a considerable proportion of cases. It is felt that these observations, establishing the predominance of B. influenzÆ in the early acute stages of the disease, are of considerable significance, especially when exactly the reverse condition was found in studying the incidence of the organism in cases of measles.

In consideration of the primary cause of influenza, attention has often been focused upon the many different bacteria found in autopsy cultures. The most prominent of these are the ill-defined diplostreptococci of the European writers, the various immunologic types of pneumococci, and S. hemolyticus. Other microorganisms less frequently found are staphylococci, M. catarrhalis, nonhemolytic streptococci, and B. mucosus capsulatus. It is not within the scope of this paper to discuss their relation to the various types of pneumonia found at autopsy, but their very multiplicity would seem sufficient prima facie evidence that they bear no etiologic relationship to influenza and must be regarded only as secondary invaders. If any further support for this opinion were necessary, it may be found in the studies upon the incidence of pneumococcus and S. hemolyticus in early cases of influenza described in this report. Both were found to occur in the same proportions in which they may be found in normal individuals at any time.

Although Pfeiffer maintained that B. influenzÆ was found only in true epidemic influenza, the incorrectness of this contention has been thoroughly established by many reliable investigators and it has been shown beyond question that influenza bacilli may always be found in a small proportion of normal individuals and are not infrequently found in other respiratory diseases.

The fairly extensive study that has been made of the incidence of B. influenzÆ in normal men and in cases of measles has clearly demonstrated that the proportion of carriers found in any group depends upon the prevalence of influenza in the group studied and that with the progress of the epidemic the percentage of carriers has steadily increased. When one considers that the opportunity for the dissemination of B. influenzÆ by contact infection is almost unlimited during an epidemic of the proportions of that which has swept over the country, this is not at all surprising. That such a large number of normal individuals became carriers of B. influenzÆ during the epidemic would seem to be sufficient evidence that actual dissemination does occur and to controvert the theory that in actual cases of influenza, conditions are established in the respiratory tract whereby B. influenzÆ, always present in small numbers, is enabled to “grow out” and become the predominant organism. From a consideration of all the observations made as to the incidence of B. influenzÆ in various conditions it would appear that the carrier condition is quite analogous to that found with many other bacteria, and may be divided into three groups: (a) acute carriers, those having influenza, (b) contact carriers, those who during epidemic times become temporary carriers of the organism without contracting the disease, and (c) chronic carriers, the relatively small number of normal individuals or those with chronic respiratory conditions who carry B. influenzÆ over long periods of time. From the facts at hand this would seem to be the most probable explanation of the conditions found. It is certainly true that the established presence of pneumococcus, B. diphtheriÆ, meningococcus and many other organisms in a varying proportion of normal individuals is not regarded as sufficient evidence to exclude them as the etiologic agents of the diseases which they cause.

It is quite obvious that if B. influenzÆ is to be regarded as the cause of epidemic influenza, it must change quite rapidly under certain circumstances from a relatively saprophytic organism to a relatively virulent pathogenic organism, and conversely return to its avirulent state following the passage of an epidemic. Animal experimentation has taught us that virulence is acquired by the rapid passage of an organism from host to host. That an opportunity for the rapid transference of B. influenzÆ from man to man was provided by the assembling of large groups of individuals relatively susceptible to respiratory diseases in our camps and cantonments is by no means impossible. It has been clearly shown by Vaughn and Palmer^[22] that men from rural districts are very susceptible to respiratory diseases and that the camps in which such men were assembled suffered most heavily in this respect during the winter of 1917–18. This Commission has clearly demonstrated that an epidemic of influenza swept through Camp Funston^[21] in the spring of 1918 and that a similar epidemic occurred at Camp Pike. Accumulating evidence will undoubtedly show that like epidemics existed in many of our southern camps (Vaughn and Palmer,^[22] Soper^[23]). It is of considerable interest that B. influenzÆ was found in almost one-half of the cases of bronchopneumonia studied by Cole and MacCallum^[24] at Fort Sam Houston in February and March, 1918. This relation is especially noteworthy, since an epidemic of influenza was seen by one of us (Blake) among the troops at Kelly Field and Fort Sam Houston during these months. That similar conditions existed in European armies as early as 1916–17 is suggested by the reports of Hammond, Rolland, and Shore^[25] and of Abrahams, Hallows, Eyre, and French^[26] on epidemics of “purulent bronchitis” with bronchopneumonia in the British army. B. influenzÆ was found abundantly in these cases.

Theoretically, under the conditions outlined above, ideal opportunities have been provided for B. influenzÆ to build up sufficient virulence to enable it to produce the pandemic of 1918–19. While it is thoroughly recognized that these considerations are in the main hypothetical, it is felt that they are by no means beyond the bounds of possibility, and for that reason are offered as suggestions worthy of further investigation.

It is, of course, perfectly possible on the basis of the observations presented still to regard B. influenzÆ as a secondary invader which makes its appearance in all cases of influenza simultaneously with the onset of clinical symptoms. Final proof of its causal relationship to the disease must depend upon the production of influenza by experimental inoculation. Results hitherto obtained in attempts to produce the disease experimentally have been contradictory. Pfeiffer^[8] claimed to have produced a disease in monkeys in some respects resembling influenza by the intratracheal injection of freshly isolated cultures of B. influenzÆ. Wollstein,^[19] in studies upon the pathogenicity of various strains, has shown that B. influenzÆ is generally pathogenic for mice and guinea-pigs without respect to source or virulence for man. Pathogenicity for rabbits and monkeys, on the other hand, was possessed only by strains that were highly virulent for man. She furthermore pointed out that for successful animal experimentation, it is imperative that inoculations be carried out immediately after the isolation of the bacilli because they rapidly lose virulence by subculture on artificial media. It is felt that failure to appreciate these facts has been responsible for the often repeated statement that B. influenzÆ is not pathogenic for animals.

In a series of animal experiments carried out by this commission recorded in an appendix to this report, sixteen-hour cultures of B. influenzÆ freshly isolated from early cases of influenza were demonstrated to be pathogenic for monkeys, both by inoculation of the nasal and pharyngeal mucosa and by intratracheal injection. Monkeys so inoculated developed coryza, epistaxis, tracheitis, bronchitis, and extreme prostration. Experiments with forty-eight-hour cultures of strains preserved by subculture during from ten to fifteen days failed to demonstrate pathogenicity for monkeys. Proof that these monkeys had influenza can depend only upon the demonstration that they suffered with a disease having the clinical character and pathologic lesions of influenza.

The reported failure to produce influenza in man by direct inoculation with freshly isolated cultures of B. influenzÆ in experiments conducted on volunteers by the United States Public Health Service^[27] at Gallops Island, Boston, is interesting, but would seem to lack definite significance since attempts to transmit the disease from man to man by direct contact also failed. Since all the subjects of these experiments had been previously exposed to influenza during the epidemic, 30 per cent actually having contracted the disease, it would seem probable that the remaining 70 per cent were only very slightly if at all susceptible. It is noteworthy that the attack rate of influenza in most army groups was approximately 20 to 30 per cent during the epidemic, the remaining 70 to 80 per cent failing to contract the disease though equally exposed. No other explanation presents itself except that influenza is no longer transmissible when clinical symptoms have appeared.

Conclusions

1. Consideration of all the evidence available makes it seem highly probable that B. influenzÆ is the specific etiologic agent of epidemic influenza, because (a) it is always present in early uncomplicated cases of influenza; (b) it is predominantly so during the acute stage of the disease in cases going on to rapid recovery without development of complications; (c) its presence in varying numbers in normal individuals and in other diseases of the respiratory tract is not valid evidence against its etiologic relationship to influenza, but on the contrary is quite in harmony with what should be expected from our knowledge of other bacteria known to be the etiologic agents of various respiratory diseases; (d) its rapidly increasing prevalence in normal individuals simultaneously with the progress of the epidemic indicates that actual dissemination of B. influenzÆ readily occurs and is very widespread during pandemic times; (e) cultures of B. influenzÆ freshly isolated from early acute cases of influenza are pathogenic for animals, and may produce in monkeys a disease closely resembling influenza.

2. Final proof of the exact relationship of B. influenzÆ to influenza must depend upon (a) more definite knowledge of the immunology both of the organism and of the disease, and (b) knowledge of the pathologic lesions of influenza and the production of these lesions in animals by inoculation with B. influenzÆ.