In the past decade, outbreaks of acute pneumonitis have been described in civilian populations,1 schools and more recently in U. S. Army camps2 which have differed from the lobar and lobular (bacterial) pneumonias in their clinical course, signs, symptoms, x-ray appearances and laboratory findings; these have been designated therefore as "atypical pneumonia" or "primary atypical pneumonia, etiology undetermined." As a rule the clinical onset of the disease is gradual, with constitutional symptoms, fever and a dry, nonproductive cough and with minimal physical signs in the chest. The changes in the chest contrast significantly with the well developed spotty lesions that can be demonstrated by x-ray films of the lungs. In short, the clinical aspects of the disease are not unlike those of a severe upper respiratory infection.
The etiology of atypical pneumonia is still obscure. Weir and Horsfall3 isolated from a human case a virus pathogenic for