144 IMMUNITY. THEORIES OF IMMUNITY 



microorganism, a minute bacillus, which he found was capable of pro- 

 ducing the disease experimentally in healthy chickens. Quite by acci- 

 dent, so it seemed, he discovered that cultures of this bacillus could, by 

 prolonged cultivation be attenuated, for when these cultures were inocu- 

 lated into chickens, the fowls did not die or suffer any ill consequences; 

 further, and what was of the utmost importance, when these same chick- 

 ens were inoculated with virulent cultures, they were found to be im- 

 mune to chicken cholera. Here, then, was the key to active immuniza- 

 tion in the prevention of disease, and Pasteur possessed the genius to 

 realize the full significance of his discovery. 



Armed with this knowledge Pasteur, and his assistants, Roux and 

 Chamberland, next studied anthrax, an infectious disease of cattle that 

 was causing a great annual loss to the farmers of France, and the bacillus 

 of which was among the first pathogenic microorganisms to be discovered. 

 It was found that prolonged cultivation of this bacillus was insufficient 

 to attenuate the cultures, as the spores were highly resistant and re- 

 tained their pathogenicity under extreme circumstances and over pro- 

 longed periods of time. 



In 1880 Touissant published a method of attenuating the bacilli by 

 heating the blood of an infected animal to 55 C. for a few minutes; 

 later, Chauveau secured similar results by heating fresh cultures for a 

 few minutes at 80 C. Both methods were uncertain, and neither safe 

 nor practical. After prolonged experimentation Pasteur found that 

 cultivating the bacilli at the relatively high temperature of from 42 to 

 43 C. resulted in gradual attentuation, and if this cultivation was con- 

 tinued, the cultures were robbed entirely of their disease-producing 

 power. Further, subcultures of these growths when kept at 37 C. 

 did not regain their original virulence, but maintained for generations 

 the grade of attenuation reached in the original culture the result of 

 cultivation for a certain number of days at the higher temperature. In 

 this manner Pasteur was able to control to some extent the degree of 

 attenuation, and by inoculating first a highly attenuated and later a 

 less markedly attenuated culture he could immunize animals against 

 anthrax. This discovery was soon amply verified. The original method 

 is practically the one employed today, and is proving of considerable 

 economic value. 



Pasteur's next great experiment was undertaken for the relief of 

 rabies, a condition in which, for the first time, he came to deal with a 

 disease that not infrequently affects man. His success and the results 

 of his discovery of an effective means of immunization against hydro- 



