In view of the existing uncertainty concerning the bacteriology of influenza and its associated pneumonias, the commission has availed itself of the opportunity afforded by the epidemic of influenza to determine what bacteria were present in the nasopharynx and sputum in these diseases. The examinations have been necessarily limited to a small proportion of the immense number of patients admitted to the hospital with influenza and pneumonia. Autopsies on those who have died with pneumonia have offered a more direct means of determining the relation of bacteria to inflammation of the bronchi and lungs. An attempt has been made to classify the pneumonias following influenza and to determine their relation to the complex bacterial flora of the injured respiratory passages. These studies have shown very early the threatening prevalence of streptococcus pneumonia, and appropriate measures have been taken to combat the spread of this infection. No better illustration could be furnished to demonstrate the value of routine performance of autopsies as a means for the recognition of obscure epidemic disease. In view of the wide difference of opinion concerning the pathology of influenzal pneumonia special study has been given to the lesions of the disease, because the epidemic has furnished the unique opportunity of examining all instances of pneumonia accurately referable to an epidemic of influenza attacking a large but definitely defined group of individuals (50,000 troops). In a civil hospital there is often great difficulty in deciding, even in the presence of an epidemic, if death from pneumonia is the result of influenza, but at Camp Pike the relation of the heightened death rate to the epidemic has excluded all save a trivial error in determining the relation of fatal pneumonia to influenza. At the direction of Col. F. F. Russell, who has promoted the work of the commission by unfailing aid, a special study has been made of the relation of hemolytic streptococcus to the complications of measles. During the later period of the investigation at Camp Pike experiments were performed on monkeys to determine the pathogenicity of B. influenzæ and of microorganism isolated from the pneumonias following influenza. Typical lobar pneumonia was produced in monkeys by intratracheal injection of pneumococci. These experiments are described in an appendix. The Surgeon General has approved the publication of this report but the authors alone are responsible for the views expressed. EUGENE L. OPIE. Washington University School of Medicine CONTENTS CHAPTER I PAGE THE ETIOLOGY OF INFLUENZA. (BY FRANCIS G. BLAKE, M.D.; THOMAS M. RIVERS, M.D.; JAMES C. SMALL, M.D.) 25 Discussion, 43; Conclusions, 49.