Relationship of Viruses to Malignant Disease

Abstract
The fact that viruses can cause cancer in a wide variety of animal species is now firmly established1-8; and the list of known animal viral tumor agents is steadily growing. Since basic biological phenomena are generally quite similar throughout the animal kingdom, it would be surprising indeed if humans were not susceptible to viral oncogenesis. Barriers encountered in investigations of animal-tumor viruses point up the exceedingly difficult problems one might expect to encounter in a search for human-tumor viruses. On the other hand, study of these agents has supplied much valuable information concerning the complex inter-play of other biological factors which influence the occurrence of virus-induced tumors. Much has been learned concerning the importance of genetic and hormonal factors. The importance of genetic factors has been best evaluated in mice because of the large number of highly inbred strains available. For example, the ability of the Bittner mammary-tumor agent