16 THE GERM THEORY OF DISEASE. 



reasoned that the action of epidemic and contagious diseases 

 was the same. From the sick person it seemed that a sub- 

 stance or force was conveyed to the well. Thus, one sick 

 person may infect a dozen, and each of these a dozen more, 

 etc., so that the disease is rapidly diffused through a com- 

 munity. It will be seen at once that the similarity between the 

 phenomena of these diseases and the phenomena of fermenta- 

 tion is very close. These two hypotheses gave rise to much 

 discussion. They seem to have stood side by side until the 

 one merged into the other, for the more they were discussed 

 the more similar they seemed to become. Finally, the first is 

 lost sight of and the Zymotic theory of epidemics and con- 

 tagions, as developed by Willis in 1659, has continued to be 

 the generally accepted theory. 



JENNER. 



When Jenner discovered the relation of the vaccine disease 

 to smallpox, and began the use of the vaccine virus as a pro- 

 tective against that malady in 1798, there came a much 

 greater confidence in the fermentation, or as it is called, the 

 zymotic theory of contagion. Here the physician had a body 

 that he could carry about and use at will, as he could yeast. 

 He could introduce this disease-producing yeast, or virus, into 

 the skin of a healthy person, and after a certain time, a disease 

 would manifest itself; just as he could introduce yeast into a 

 solution of sugar, and after a certain time, find the phenomena 

 of fermentation. A small amount of either was sufficient ; in 

 each case a certain period of rest was observed before the 

 characteristic manifestations declared themselves ; a stage of 

 incubation. In each case the peculiar product first used was 

 largely increased. In case of the vaccine, a dozen could be 

 vaccinated from the product of the first vaccination, just as 

 from the product of the fermentation of a single butt of must 



