Abstract
After a brief comment on the history of the surprisingly large number of discoveries of new viral agents that have been made during the last 25 years, certain factors considered to be responsible are discussed. Recent progress in the field is then illustrated by summaries of the present status of investigation relating to (1) Coxsackie viruses; (2) ECHO viruses; (3) Respiratory tract viruses ("adenoviruses"); (4) Viruses closely resembling measles virus and lately isolated from cases of giant cell pneumonia at autopsy.