IV. THE VARIABILITY OF VIRUSES 163 



infection may be involved. Margulis et al. (78) have claimed that 

 they were actually able to isolate a virus from a patient of postinfec- 

 tion encephalitis. The agent causing postvaccinal encephalitis may 

 either be present in normal animal cells or produced by the stimulus 

 of the inoculated virus. 



3. Increase and Decrease in the Virulence of Viruses 



It is generally accepted that when a virus is propagated serially 

 in a host different from the customary, it increases gradually in its 

 virulence toward the host. For example, an influenza virus isolated 

 from a man fails usually to produce no lung lesions in hamsters, 

 although it can readily multiply in the animal, but on serial passage 

 it will acquire the capacity to cause pulmonary consolidations and 

 even the death of the animal (79). If rabies virus isolated from a dog 

 is inoculated into a rabbit, the incubation period of the disease is 

 about a fortnight, but if the virus is passed in succession through a 

 series of rabbits, the incubation period gradually falls, till after 20 to 

 25 passages only to 8 days, and after further 20 to 25 passages it is 

 no more than 7 days. Such a virus has a greater affinity for the 

 nervous system of the rabbit than the virus isolated from the dog. 



In the concept of the writer, such an enhancement in the viru- 

 lence should result mainly from the increase in the combining force 

 between a virus and the host cell. Namely, the virulence is in- 

 creased because the combining force is increased when a virus is 

 passed in succession through a series of the same kind of host. The 

 reason for the increase in the combining force is considered as follows : 

 The protoplasm of the host cell and the virus affecting the host are 

 both assimilases, though the latter is stronger since it acts as a virus 

 upon the host. However, the host protoplasm, though its assimilase 

 action is weaker than that of the virus, will be able to exert some 

 influence upon the latter, as the protoplasm is overwhelmingly larger 

 in quantity than the virus, so that in the long run the virus itself 

 may be assimilized by the host protoplasm to some extent in case 

 w^hen the virus is passed successively through the same kind of host, 

 and as a result the protoplasm and the virus n'.ay come to share a 

 structure or structures in common ; namely, they may become gradu- 

 ally to resemble to each other. On the other hand, as already discus- 

 sed, the development of common structure is nothing but the produc- 

 tion of combining force between them, and therefore the affinity of 

 the virus for the host cell may be enhanced with the increase in the 

 virulence on continued passage through the same kind of host. 



