Facts and theories on viruses causing cancer and leukemia.

It is possible to explain the familial incidence of cancer developing in several members of the same family tree, within the same as well as in successive generations, by an assumption that tumors and leukemia are caused by oncogenic viruses transmitted in a latent form from one generation to another in many animal species, presumably also in man. The term "vertical transmission" was coined to describe this form of transmission of pathogenic agents. In most instances the oncogenic viruses are invisible and harmless to their carrier hosts. Occasionally, however, under the influence of endogenous or exogenous inducing factors, these viruses become activated and cause cancer or leukemia. According to this concept, the law of obligate communicability established for all common infectious agents applies also to oncogenic viruses: each tumor or leukemia can be traced to another similar tumor which developed in one of the preceding generations as a result of "vertical" passage of the same oncogenic virus. This concept postulates that at one time, perhaps centuries ago, these viruses entered from outside the animal hosts and that, since then, they have been transmitted from one generation to another. The theory of vertical transmission of latent oncogenic viruses is consistent with experimental and clinical observations made during the preceding two decades. We consider this concept to be a logical and most promising approach to the problem of viral etiology of neoplastic diseases.