CHAPTER V 

 FIXED VIRUSES 



1. Immunity against Viruses 



Usually an effective and lasting immunity is afforded by a single 

 attack of a virus, a fact which is sometimes regarded as one of the 

 characteristics of viruses. Small pox, yellow fever, varicella, measles, 

 mumps, cattle plague, swine fever, and dog distemper, all these are 

 common virus disease, and all confer an immunity that, in the majori- 

 ty of cases, appears to last throughout life. In some virus diseases, 

 however, such as influenza, common cold, and poliomyelitis, the im- 

 munity is believed to be only transient. 



As above mentioned, viruses such as those of common cold, in- 

 fluenza, and poliomyelitis, appear to be generated de novo. From this 

 point of view, it may be able to explain the transient nature of the 

 immunity against these viruses, because the immunity against a virus 

 formerly produced would be ineffective to a newly produced virus. 

 Not long ago foot-and-mouth disease would have been placed in this 

 non-immunizing or poorly immunizing class, but we are now informed 

 that there are at least 3 immunologically distinct types, and that 

 animals that have recovered from infection with one type proves im- 

 mune to further infection with the same type, but are readily infected 

 by the other types. 



Experimental results with one and the same strain of influenza 

 virus showed, however, that the immunity is actually transient. For 

 example, ferrets that have recovered from an attack of a strain of the 

 virus are, for a time, resistant to the infection, but this persists only 

 for about 3 months, and then gradually wanes. Again, the immunity 

 which develops following an attack in man is said also not to last for 

 long periods, usually becoming insignificant after 6 to 8 months (80). 

 It must, therefore, be admitted that the immunity in itself is actually 

 transient in some viruses, although sometimes the possibility of their 

 successive generation may account for the apparent transient im- 

 munity as above pointed out. It is, however, a very remarkable fact 

 that immunity appears incomplete in only the viruses that can be con- 

 sidered to be generated de novo such as those of influenza, and 

 poliomyelitis. 



