326 CLINICAL BACTERIOLOGY. 



take place through the mouth or the lungs. Naturally, 

 however, under any circumstances other modes of infection 

 can not be excluded. 



Predisposition and Immunity. The susceptibility to 

 smallpox exists at all ages. Even new-born children 

 have been observed to present the disease. In the great 

 epidemics of smallpox that traversed Europe before the 

 institution of compulsory vaccination the morbidity varied 

 in different localities. The local predisposition was the 

 greater the more impoverished the community. It thus ap- 

 pears that not merely the telluric conditions themselves, but 

 rather the vital conditions and the mode of life of the people 

 living upon a given soil, constituted the cause for the varying 

 distribution of the disease. The marked increase in small- 

 pox that regularly takes place in winter has been considered 

 evidence of a temporal predisposition. This may, however, 

 be readily explained by the changed mode of life in winter 

 (with greater confinement in closed rooms, the wearing of 

 heavier and a greater amount of clothing, the greater 

 difficulty of cleanliness, etc.), without the necessity for in- 

 voking a direct influence of the weather upon the germ of 

 the disease. 



Recovery from an attack of smallpox confers immunity 

 that lasts on the average about ten years. A second attack 

 of the disease before the expiration of ten years is most 

 exceptional, but has been frequently observed after a 

 longer period. Third attacks of smallpox have been re- 

 ported nine times in the literature, and Cantani reports one 

 case in which seven attacks occurred. 



Upon the ancient experience that immunity is acquired 

 by recovery from smallpox is based the procedure of vario- 

 lation that is, the inoculation of healthy persons with true 

 smallpox for purposes of immunization, which was formerly 

 practised and entailed not a few sacrifices. 



Vaccinia and Vaccination. Edward Jenner, a physician 

 of Berkeley, in England, between 1749 and 1823, con- 

 vinced himself, after study and investigation pursued for 

 years, that the pock-disease of cows (vaccinia), conveyed 

 to human beings, protected them from infection with variola. 

 On May 14, 1796, Jenner vaccinated a youth with cowpox- 

 lymph obtained from the hand of a maid who had infected 

 herself in milking a cow suffering from cowpox of the 

 udder. The vaccinated subject was protected against sub- 



