4 INFECTION AND IMMUNITY. 



tagious disease: "An epidemic disease can orig- 

 inate and extend only through the agency of the 

 germ producing it." Yet at this time no infec- 

 tion had been definitely proved to be of microbic 

 origin. 



In 1850 Eayer and .Devaine made an observa- 

 tion, which might have fallen into the oblivion of 

 many preceding ones had it not been confirmed by 

 later investigators. They found "small filiform 

 bodies" in the blood of sheep which had died of 

 anthrax, and were naturally inclined to believe 

 that these forms caused the disease. Other scien- 

 tists, especially Pasteur and Koch, soon took up 

 the study of anthrax, with the result that the 

 small rods of Devaine were scientifically proved to 

 be its cause. 



Two great minds dominated medical research 

 at this time Pasteur and Koch. Pasteur, in his 

 early career as a chemist, had had his attention 

 called to the processes of fermentation. He re- 

 curred to this subject at a time when the theory 

 of the spontaneous generation of small living 

 forms was widely discussed, and in 1857-1861 

 proved beyond any possibility of doubt that lactic 

 acid, alcoholic and butyric acid fermentations are 

 due to the action of minute living cells ; and, fur- 

 thermore, that each particular kind of fermenta- 

 tion has its own peculiar microbe as the cause. 

 This was an example of what AVC term to-day mi- 

 crobic specificity, in marked contrast to views 

 w ^i cn were then prevalent regarding the variabil- 

 ity of micro-organisms.* Pasteur then applied 



* "The following citation from Nageli illustrates clearly 

 this idea of unlimited variability of microbes : 'In the 

 course of generations the same species assumes alternat- 

 ingly different morphological and physiological forms which, 

 as years and periods of years pass by, may cause now 



