In the past few years numerous clinicians have described isolated cases and epidemics of infection of the respiratory tract especially associated with pulmonary involvement. The disease in most instances resembled epidemic influenza so closely that clinical differentiation could not be made, and in only one or two studies were attempts made to differentiate the disease from influenza by serologic or other laboratory methods. Definite diagnoses in this group of infections of the respiratory tract cannot be made, of course, until the etiologic agents in the different diseases become known. Various names have been given to the diseases described, such as acute pneumonitis,1acute influenza pneumonitis,2acute interstitial pneumonitis,3atypical pneumonia,4bronchopneumonia,5pseudolobarbronchopneumonia,6benign circumscribed pneumonia,7disseminated focal pneumonia,8acute diffuse bronchiolitis9and capillary pneumonia. These terms may be fairly descriptive of the underlying pathoanatomic changes, but beyond this they serve no useful purpose. Furthermore, in our recent experience with a