11 



serted under the skin of a healthy man. After a time a number 

 of vesicles appear upon his body, each of which is capable of 

 yielding much more virus than he received, and if he happens 

 to mix freely in a community, it will not be long before many 

 more persjons are afflicted in the same way, each one develop- 

 ing a larger or smaller number of vesicles ; every one filled 

 with the same poisonous material^ so that at the height of the 

 epidemic the poison has been multiplied millions upon 

 millions of times. In seeking for an explanation of this, it 

 was noticed that analogous phenomena were to be seen in 

 the process of fermentation. " A little leaven leavens the 

 whole lump." A very small portion of fermenting material 

 will in time set the whole mass in fermentation, and any small 

 portion of this will start a similar process if again applied to 

 fresh material. People saw that the action of Small Pox 

 poison and similar diseases was so like that of a ferment that 

 they considered these disorders were due to a kind of 

 special fermentation or Zymosis. This theory which came 

 so near to the truth was, nevertheless, afterwards abandoned. 

 The expression is, however, still used, the term Zymotic 

 Diseases having been introduced by Dr. W. Farr, I think, 

 in the year 1841. 



When a previously healthy man, after contact with a 

 patient sick of a contagious disease, subsequently develops 

 in himself an identical malady, it is only reasonable to 

 suppose that something passed from the sick man to the 

 healthy one, which set the process going. This hypothetical 

 something, this poison, has hitherto eluded the grasp of all 

 seekers. It received at one time the name of materies 

 morbosa or materies morbi ; however, by whatever name it 

 was called, it was only known by its effects. As time went 

 on fresh facts came to light. It was noticed that the 

 infecting material, whatever it was, could not only pass 

 directly from one individual to another, but that it could be 

 conveyed long distances, and that it retained its potency for 

 long periods. People in this country were infected with 

 diseases through materials brought from the other end of 



