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artificially in many more persons than would have caught it naturally, 

 had inoculation never been thought of. So that while the relative 

 mortality, i. e., the percentage of deaths from smallpox, was lessened 

 by this practice, the absolute mortality was fearfully increased." 



It was noticed, moreover, that, contrary to expectation, persons 

 who had been variolated occasionally themselves became centres 

 of infection. Evidently the attenuation of the organism by skin 

 passage was not always sufficient to make variolation an altogether 

 harmless procedure. The underlying principle, however, is evidently 

 sound, and for all ages to come these early attempts at protective 

 inoculation will form one of the great turning points in the history 

 of medicine. 



The next step in advance is intimately associated with the name of 

 Jenner. Led by the popular belief which was prevalent in Gloucester- 

 shire during the latter half of the eighteenth century, that individuals 

 who have accidentally become infected with cowpox were thereby 

 protected against smallpox, Jenner actually put this idea to the 

 test (1796). 



To this end he inoculated a healthy boy, of eight years, with 

 material taken from a cowpox vesicle on the hand of a dairy maid, and 

 a couple of months later showed by inoculation with smallpox virus 

 that the child was actually immune. In 1798 he furnished further 

 proof that cowpox will furnish protection against smallpox by 

 inoculating, or, as we may now say, vaccinating (from vacca, the 

 cow), a child directly from a cowpox vesicle, and continuing the 

 inoculation from arm to arm through a series of five children, after 

 which all five were variolated, i. e., inoculated with human smallpox, 

 the result again being negative. 



Hereafter vaccination was extensively practised in different 

 European countries, and also introduced in America, the source of 

 material for a long time being lymph which was obtained from cows 

 that had developed cowpox, and in some instances from horses, 

 affected with grease, the affections having been shown to be identical. 

 While Jenner and many later investigators failed to recognize the 

 identity of human smallpox with cowpox, as well as grease, this 

 seems now to have been satisfactorily established, the apparent 

 differences between the two conditions and the effect of the inocula- 

 tion of the two kinds of virus being the result of the attenuation of 

 the organism in question, in consequence of animal passage. 

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