On the Nature of Cancer 125 



developments now compel serious consideration of the proposition that 

 all forms of cancer are cellular reactions to infectious processes. 



Viruses have already been implicated in a number of other animal 

 cancers, including leukemia, which also has a human parallel. The 

 techniques employed in the animal-tumor experiments, however, are 

 not applicable to human beings since cancer viruses like the cancers 

 they cause are species specific and variable in form and behavior. 

 Perhaps the only applicable property common to all viruses is that 

 they propagate. Fortunately the intact animal host is not essential; 

 suspensions of living cells, tissue cultures, can support the growth of 

 viruses in many instances. It is important to distinguish between 

 the cause of a particular form of cancer, a virus perhaps, and the 

 cellular reaction to that virus infection, the cancer. Often both can 

 be separately cultivated, transplanted, and observed. Tissue culture, 

 now in the full bloom of its renaissance, appears to offer the means for 

 elucidation of the role of viruses in human cancer and also of the 

 metabolic deviations which are the inherent characteristics of cancer. 

 Experiments are in progress wherein human secretions are admixed 

 with tissue cultures on the expectancy that carcinogenic viruses, if 

 present, will reveal themselves either by increase in number of particles 

 or by malignant transformation of host cells. 



It is intrinsic to our virus hypothesis that the presence of the virus 

 in the cancer cell is wholly fortuitous, since the mutation produced by 

 virus aggression is self-perpetuating. Although the malignant trans- 

 formation appears to be an escape mechanism the malignant cell itself 

 is not necessarily unpropitious for viral existence. Porter 14 was able 

 to obtain the beautiful electron micrograph reproduced as Fig. 2 by 

 tissue culture of mouse mammary carcinoma cells in which the virus 

 was still present. This picture demonstrates the exuberant growth of 

 the virus in a cell. Note the resemblance to the isolated particles 

 of Fig. 1. 



Now, Bittner's informative foster-nursing experiments established a 

 procedural pattern for other workers who were in possession of inbred 

 strains of animals with high incidence of one form of cancer or another. 

 MacDowell of the Carnegie Institution had been investigating the 

 genetic aspects of leukemia which was endemic in his C58-strain mice 

 when he undertook to test the possibility of milk transmission of the 

 disease by reciprocal foster nursing with Storrs-Little strain which was 

 free of leukemia. His first experiments were inconclusive, and so he 

 stubbornly repeated the experiment, this time with larger numbers. 

 In so doing, however, it was necessary for him to employ all of the 



