SMALLPOX 171 



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

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

 mortality, /. r., 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 w r as 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 later 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 ma- 

 terial 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 Euro- 

 pean 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 inoculation of 

 the tw r o kinds of virus being the result of the attenuation of the 

 organism in question, in consequence of animal passage. 



