122 PASTEUR 



Pasteur further noted that the bacillus was not 

 equally fatal in all animals, and that it changed its 

 character when passed through the body of certain 

 classes of animals. It was, however, not in studying 

 the Bacillus anthracis that he made the far-reaching 

 discovery of the attenuated virus. This he first noted 

 when at work on chicken cholera, a disease very fatal 

 in poultry-yards; and he made the important discovery 

 by one of those happy accidents which only occur to 

 those who possess the genius for observation. During 

 his numerous experiments he one day chanced to in- 

 oculate some fowls with a forgotten culture some 

 weeks old. To his surprise the chickens, though 

 made ill, did not succumb ; in fact, they rapidly re- 

 covered. He immediately tried what the effect would 

 be if these same fowls were inoculated with fresh 

 cultures of a kind so powerful as to be undoubtedly 

 fatal to a healthy bird which had never suffered from 

 the disease. To his delight, the inoculated fowls 

 resisted the poison, and proved, in fact, immune. This 

 simple experiment is the basis of the world-wide pro- 

 phylactic measures which are now being carried on 

 against all forms of bacterial disease ; and, although 

 Pasteur's explanation of the weakening of the virus 

 which he attributed to oxygenation has been shown 

 to be erroneous, he must still be regarded as the 

 originator of methods for the production of immunity 

 by means of artificially attenuated organisms. 



If the virus of chicken-cholera can be attenuated, 

 and when attenuated produces immunity from later 

 attacks, the same is probably true of other germs 

 which can be cultivated outside the body. Arguing 

 in this fashion, Pasteur returned to his study of 

 anthrax. Here he also succeeded, and in the spring 

 of 1 88 1 he demonstrated the value of his treatment. 

 Out of a flock of fifty sheep one-half were inoculated, 

 the other half were not; the whole flock was then 



