INFECTION AND IMMUNITY 363 



failure and being timid by nature, it was not until May 

 14, 1796, that Jenner gave a public demonstration. A 

 boy was inoculated with cow-pox, and having passed 

 through the disease, was found upon inoculation to be 

 immune against small-pox. In 1798, Jenner wrote 

 a lengthy paper upon " vaccination," detailing the whole 

 matter and stating his belief and his proofs. For a long 

 time the medical profession as well as the laity were 

 incredulous, but experiments were made by one after 

 another of the prominent physicians and the method 

 slowly spread until its advantages became so apparent 

 that it became adopted in one after another of the 

 European countries and finally in America, ultimately 

 being made more or less compulsory in all countries 

 with such striking success that small-pox, once the most 

 frequent and most terrible of maladies, has become a 

 comparatively rare disease in most civilized countries. 



From the fact that the disappearance of small-pox 

 has been coincidental with the disappearance of cow-pox, 

 swine-pox, etc., and that the inoculated viruses of 

 these diseases afforded protection against small-pox, 

 there can be little doubt that these affections had a 

 common ancestry, and that the mild character of 

 vaccinia, or cow-pox, in man is due to some change 

 suffered by the specific microparasites a diminution in 

 their virulence resulting from their exposure to the 

 defensive juices, etc., of the cow. 



No further progress was made in the field of experi- 

 mentally induced immunity from the invention of 

 vaccination by Jenner until the time ^of Pasteur, nearly 

 one hundred years. 



With Pasteur, however, came a new epoch, that of 

 the discovery of the microparasites of disease, and shortly 

 after, through the work of Koch and Pasteur, the means 

 of artificially cultivating and accurately observing them, 

 and in consequence a great expansion in the knowledge 

 of infectious diseases and in the means of preventing 

 them. 



