150 III. THE EVOLUTION OF VIRUSES 



conditions under which the volunteers were left, for it is a very well 

 attested fact that among a group of people isolated on a remote island, 

 particularly if the community is small, common cold tends to die out. 

 The experiments were reported to have been carried out in a solitary 

 environment. The volunteers in this case, therefore, may be comparable 

 to the flies bred by us in laboratory. In both cases some factor may 

 be lacking against the virus production. According to the English 

 workers, although common cold was induced in the volunteers when 

 the virus was administered, the symptoms were unusually slight, 

 mostly without fever, recovering in two or three days, and about 

 50 per cent of the volunteers did not show any symptoms. This may 

 remind one of the fact above mentioned that phage harbours only for 

 short periods, when the virus is administered to the reared flies. 

 Thus the factor necessary for the virus production may be needed also 

 for the thriving of the virus in the host cells. 



It should be remembered in this connection that poliomyelitis virus 

 and also Coxackie virus, which is closely related to the former, are 

 very frequently isolated from flies, suggesting that these viruses, like 

 phage, may be produced in flies. A number of evidences can be pre- 

 sented to suggest that epidemic poliomyelitis cannot be explained on 

 the basis of person to person contact, but that the epidemic seems 

 rather to be induced by a common source. Attention should be paid 

 on the fact that the seasonal concentration of healthy virus-carriers, 

 especially virus-carrying flies, is proved in the area where the disease 

 is prevailed (49). 



Like phage, both poliomyelitis and Coxackie virus fed to flies bred 

 in laboratory not only fail to multiply in the flies but also fail to be 

 retained in the insect body for long periods, suggesting that only 

 naturally living flies can produce the viruses (50). 



Rous sarcoma can be induced by the stimulus of tar, and it can be 

 sometimes transmitted by a virus-like agent, whereas the action of 

 the latter is remarkably enhanced by the simultaneous application of 

 tar. A factor or factors like tar in this case may be lacking in the 

 above case of common cold, whose virus may be unable without such 

 a factor to exhibit its full action and at the same time the virus it- 

 self may not be produced. The virus, however, may be incapable of 

 thriving indefinitely even when the factor is present, but gradually 

 disappearing, and the virus may be generated de novo in succession. 



