ANTHRAX AND CHICKEN CHOLERA 15 



Pasteur would not have thought of trying it if he had 

 not made an accidental discovery along the same line 

 several years before. In the early '70's he had been work- 

 ing with a disease of poultry known as chicken cholera, 

 which he had concluded was caused by some microorgan- 

 ism. He had a culture of this organism, in fact, and used 

 it again and again to produce the disease in healthy fowls. 

 But one time, on returning to his laboratory after a vaca- 

 tion, he foimd it had lost its power of causing the disease ; 

 but before learning the fact he had inoculated several 

 chickens with it. He then obtained a fresh culture, and 

 much to his surprise discovered that although it would 

 produce the disease in other chickens, it had no effect on 

 those which had been inoculated with the old culture. 

 This was the accidental observation that has nearly revo- 

 lutionized the practice of medicine. To a mind less keen 

 than Pasteur's it might have merely suggested that these 

 particular chickens were "tough old birds" not susceptible 

 to the disease. But no such explanation satisfied Pasteur ; 

 he wondered why they were not susceptible, and imme- 

 diately made the right guess — ^they had become immune 

 because he had first inoculated them with an old culture, 

 one too weak to cause the disease. Thus he concluded 

 that an animal could be made immune to a disease by 

 inoculating it with a weakened culture of the organism 

 causing that disease. That is probably what is done in the 

 case of smallpox vaccination. Cowpox is probably a modi- 

 fied form of smallpox, weakened by passage through cattle 

 and no longer capable of giving a human being a real at- 

 tack of the disease, but capable of making him immime. 

 This possible identity of cowpox and smallpox is stiU noth- 



