90 Immunity 



success of his work immediately attracted the attention 

 of both scientific investigators and sanitarians, and its out- 

 come has been the establishment of compulsory vaccination 

 by legal enactment in nearly all civilized countries, with 

 the result that smallpox, instead of being one of the most 

 prevalent and most dreaded diseases, has become one of the 

 most rare and least feared. 



The immunity acquired through vaccination is active and 

 usually of prolonged duration. It is subject to the same 

 variations observed in other experimentally acquired im- 

 munities, these variations explaining the occasional failures 

 which constitute the "stock in trade" of those who still 

 remain unconvinced of the scientific basis and efficacy of 

 the procedure. 



Though a thorough analysis of the irregularities and ex- 

 ceptions of vaccination would be of much interest, a brief 

 mention of the most important must suffice for the present 

 argument. 



The first controversial point is the nature of the "vaccine, " 

 or virus used in the operation. It is obtained from calves 

 or heifers suffering from experimental cow-pox, and is a virus 

 descended from various spontaneous cases of cow-pox 

 observed in places remote from one another. Experts are 

 undecided whether cow-pox is variola modified by passage 

 through the cow so that the transplanted micro-organisms 

 are only capable of inducing a local instead of a general 

 disease, or whether it is an independent affection natural to 

 the cow. 



In reality the matter in unimportant, so long as the desired 

 effect is accomplished, and the true lineage of the virus is 

 only a matter of scientific curiosity. As immunity is almost 

 invariably a specific effect resulting from infection, it would 

 seem most likely that cow-pox and smallpox were originally 

 identical. 



The advantage of "vaccination" over "inoculation" is 

 that the induced disease is local and not dangerous except in 

 rare cases, and that it is not contagious. The natural varia- 

 tions in the susceptibility of different vaccinated individuals 

 determine that a few persons cannot be successfully vacci- 

 nated, being immune to the mildly invasive organisms of 

 vaccinia, though perhaps susceptible to the actively in- 

 vasive organisms of variola; that a few individuals shall 

 prove abnormally susceptible to vaccinia so that the disease 



