II. THE GENERATION OF VIRUSES 145 



predisposition to produce the virus are necessary. It is known that 

 there are virus-free strains of mice which never produce the virus 

 throughout their life. 



Cancer is, in a sense, a virus disease. As is well recognized, for 

 its production above cited two factors, /. e. predisposition and stimulus, 

 are indispensable. The important significance of predisposition has 

 been well realized in the case of mammary cancer of mice, in the 

 generation of which a virus is believed to be involved. Likewise in 

 the case of fowl leucosis, a kind of malignant tumour caused by a 

 virus, the predisposition of chickens plays an important role. It has 

 been reported that in a certain line of chickens the disease is almost 

 completely absent, whereas in some other lines over 50 per cent 

 are afflicted by the disease (40). 



Common cold is undoubtedly a virus disease, and as is well known 

 the disease is established when a causative factor, chilling, is effected 

 to man predisposed to catch cold. It merits attention that herpes 

 febriles, known to occur following febrile diseases such as pneumxonia 

 and malaria, may also be induced by artificial application of heat, 

 such as infra-red ray or hot water bath. Moreover, poliomyelitis is 

 liable to occur after trauma of various sorts especially follow;ing ton- 

 sillectomy. For the occurrence of these virus diseases it is needless 

 to say that predisposition is also an essential factor. But it is com- 

 monly supposed that in such cases latent viruses are activated by the 

 stimuli to cause manifest infections. This may indeed be an explana- 

 tion more reasonable than to assume that viruses are newly generated 

 by the stimuli, as will be fully discussed in Part V, but according 

 to the writer's theory latent viruses are not the viruses themselves. 

 They are present in a state of "provirus" before activation; "provirus" 

 is the protoplasm structure able to be changed into virus structure by 

 adequate stimuli. In short, by the term "provirus" is meant predis- 

 position. In this connection, it should be mentioned that sometimes 

 an active virus itself is enhanced in its activity by the presence of 

 an adequate stimulus. 



For instance, it is stated that when the leaves of tobacco plant 

 are rubbed with carborundum the number of local lesions produced by 

 tobacco mosaic virus increases approximately to such an extent as if 

 the virus concentration were increased a hundred-fold. Potato virus A 

 is sap-inoculable to potato only when carborundum is used (39). It is 

 also stated that when the virus of Rous sarcoma is injected into the 

 blood of a fowl, the virus seems to become distributed throughout the 

 blood and organs in the body but does not produce tumours. If, how- 

 ever, a muscle is injured immediately after the intravenous injection 

 of the virus, then a tumour develops at the part of injury, but if the 



