During the past century references have repeatedly been made to the possible infectious etiology of leukemia. These references have, by necessity, been conjectural and, for the most part, based on similarities between the clinical picture seen in leukemias and in infections. Attempts were made from time to time to transmit human leukemia to lower animals and even to man but these attempts were uniformly unsuccessful. Gradually, the fashions changed and the possibility that leukemia was an infectious disease was largely ignored.
A renaissance of interest in leukemia during the last 30 years has been prompted by the development of highly inbred lines of mice in which high and low leukemic strains have evolved as a result of selective breeding. Although at first the differences in incidence of leukemia were thought to be due to genetic factors, it was eventually shown that, although genetic factors do play a role, infection is