Is Leukemia Caused by a Transmissible Virus?
1. Leukemia, like other malignant tumors, has long been observed to occur more frequently among members of certain families than in the average population. This observation has been made not only in chickens, mice, and cattle, but also in humans. 2. Direct experimental evidence is now available suggesting that at least in chickens and mice, leukemia is caused by a filterable, submicroscopic agent, presumably a virus, transmitted from one generation to another directly through the embryos. 3. Although no experimental evidence is yet available concerning the nature of leukemia in cattle and in humans, the possibility should be considered that, as in chickens and mice, leukemia in other species may also be caused by a transmissible agent. In cattle, numerous observations have been made of leukemia developing in several members of the same family, in two or three successive generations. In humans, one case has been published in which three, and perhaps four, out of seven, and another in wh
