The subject of epidemic pneumonia has been receiving a great amount of attention since the recent epidemics of streptococcus pneumonia and influenzal pneumonia. There are still many unknown factors of pathogenicity, virulence, invasiveness and toxicity of microorganisms which produce epidemic diseases. These terms commonly are used without clear distinction of meaning.1 The difficulty of reproducing characteristic epidemic lesions or even causing infection in animals with cultures of highly specialized strains of bacteria is well known. In view of these difficulties in experimental reproduction of an epidemic disease, it has seemed possibly more profitable to study the natural processes of an animal epidemic of pneumonia, followed through its entire course of development and subsidence.
The opportunity to observe and study such an epidemic presented itself in the animal room of the U. S. Naval Hospital, Chelsea, Mass., during the late winter and early spring of 1919. Distemper and epidemic pneumonia in