V. FIXED VIRUSES 167 



to the one group which may be produced de novo, and those of mea- 

 sles, mumps, small pox, etc. belong to the other ; spontaneous gene- 

 ration may be impossible in this latter group. 



2. The Fixation of Viruses 



In general, viruses can be changed in their character, sometimes 

 even inactivated completely, without losing their immunological spe- 

 cificity. For example, vaccinia virus is evidently different in some 

 aspect from the variola virus from which it has been derived, but im- 

 munologically similar, so that it is capable of immunizing against the 

 latter. Phage treated with such agents as formaldehyde, mercuric 

 chloride, etc., may be reduced in its virulence, but may retain its im- 

 munological specificity even after the complete inactivation, although 

 too severe inactivation may lead to the destruction of the antigenicity 

 itself (68). This fact indicates that the structure involved in the im- 

 munological specificity is more stable than that concerned in virus 

 activity. Consequently it is supposed that the variation in the activi- 

 ty may develop without difficulty in contrast to the variation in the 

 immunological specificity. 



It may, however, be possible that a virus generated de novo can 

 be changed in its immunological specificity as well as in other charac- 

 ters to become in the long run a virus entirely different from the 

 original. It is actually confirmed that also in immunological proper- 

 ties variations can occur. For example, evidence has been presented 

 that antigenic variations take place in a strain of influenza virus pro- 

 pagated through continuous passage in tissue cultures and eggs as 

 compared with a line of the same strain maintained in mice (85). 

 Through such variations, a virus may acquire the properties differing 

 in every respect from any other viruses which can be generated de 

 itovo. 



In this way, the viruses that can induce lifelong immunity might 

 have been developed. There are good reasons to suppose as will be 

 detailed later that viruses such as those of measles and sm.all pox, in 

 contrast to those of influenza and poliomyelitis, are neither generated 

 de novo nor lost in short periods. The writer proposes therefore the 

 name "fixed viruses" to designate the former group of viruses. The 

 reason why the viruses have acquired the fixed character and why 

 they can produce rigid immunity will be fully discussed in later 

 chapters. 



It cannot of course be considered that these fixed viruses are all 

 the same in their ages ; some may be young, rather fixed recently. 



