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.