28 MODERN BIOLOGIC THERAPEUSIS 



applicable to other infections. Acting on this 

 theory he was able to immunize fowls against 

 chicken cholera. He also developed methods of 

 vaccinating against anthrax and swine erysipe- 

 las, each instance requiring some special method 

 of attenuating the virus. He found that he was 

 able to diminish the pathogenicity of parasites 

 for their natural host, not only by cultivation 

 and preservation under unfavorable conditions, 

 but also by repeated passage through other ani- 

 mals ; and that, while passage through some spe- 

 cies might diminish the virulence, passage 

 through other species might enhance it and mod- 

 ify the type of disease produced and the length 

 of the incubation period. The attenuation of 

 the rabies virus which occurs by passage of 

 the virus through rabbits gave Pasteur a 

 means of combating street rabies, which has a 

 long incubation period, by immunizing the sub- 

 ject after infection. In 1885 after many experi- 

 ments on animals, he made his first inoculations 

 in man with success ; and in 1892, Haffkine de- 

 veloped a similar method of vaccinating against 

 cholera with the living spirilla attenuated by 

 long culture. 



Basis of Vaccination All the methods intro- 



