166 III. THE EVOLUTION OF VIRUSES 



At present, influenza virus is divided into two distinct and im- 

 munologically unrelated serological types, termed A and B. However, 

 it is of much importance that individual strains of influenza A virus 

 or of B virus are not identical immunologically with other strains of 

 the same type. It is said that, in occasional instances, the immunolo- 

 gical differences between individual strains which belong to one type 

 are so great as to cause considerable difficulty in the proper identifica- 

 tion and classification of a strain. It may be said that these two 

 types may only be an artificial product of classification. 



Various strains of virus causing symptoms like those of encepha- 

 litis are also known, and new strains are constantly increasing. 

 Normal or pathological structure of the protoplasm of a certain animal 

 may be called influenza virus when it is capable of giving rise to 

 symptoms like those of influenza because of its special aflJinity for re- 

 spiratory tract of some other animals, whilst when it is endowed with 

 special aflSnity for central nervous systems, it may be called encepha- 

 litis virus. It seems that comparatively little is known of the dura- 

 bility of the immunity against encephalitis. 



Poliomyelitis may be concerned with agents belonging to this 

 category. It is certain that there are a vast number of different 

 strains in poliomyelitis virus. Recent observations suggest that there 

 are at least three main types (81) (82). As already pointed out, polio- 

 myelitis-like viruses have frequently been isolated from various sources 

 especially from flies apparently having no connection with the disease. 

 It has been known that some immunity is established on the infection 

 with this disease, but that it is only transient and inconsiderable (83). 



The so-called Coxackie virus appears to be closely related to this 

 virus. Its strains are known to be very numerous, and 16 antigeni- 

 cally distinct types have been reported (84) and like poliomyelitis virus 

 it has frequently been detected in flies. Within the past decade, it 

 has become established that during typical summer epidemics of polio- 

 myelitis the causative agents, frequently also Coxackie-like virus, can 

 be isolated from several different species of flies. 



Whatever the reason may be for the presence of the temporary 

 nature of the immunity, there seems no doubt that spontaneous gen- 

 eration is impossible in such viruses as those of small pox, mumps, 

 measles, etc., which confer a lifelong immunity, since, if new viruses 

 which could cause a similar disease were successively generated, the 

 immunity would not appear to last effectively, for new viruses would 

 be more or less immunologically different from the previous ones, 

 though they could cause a similar disease. 



It appears, therefore, reasonable to conclude that there are at least 

 two groups of viruses. Those of influenza, poliomyelitis, eto. belong 



