172 MANUAL OF BACTERIOLOGY. 



organisms causing small-pox and vaccinia have been isolated 

 in pure culture. Their identity and mode of action may then 

 be determined. 



Small-pox has been inoculated into calves and passed through 

 other calves in succession, producing finally an eruption indis- 

 tinguishable from cow-pox. Lymph taken from such calves 

 has been used successfully to vaccinate children. Not only 

 does cow-pox protect against small-pox, but it has been shown 

 that small-pox protects against cow-pox. 



Immunity Produced by Inoculation with Bacteria of 

 Diminished Virulence. — Pasteur conceived the idea of 

 attenuating the virulence of the bacilli of fowl-cholera by 

 prolonged exposure to the air. He made use of the attenuated 

 virus as a vaccine against the disease. 



A nearly similar principle was shortly afterward apphed by 

 him to the preparation of a vaccine against anthrax. When 

 anthrax bacilli were cultivated at a temperature of 43° C, 

 Pasteur obtained bacilU of very sHght virulence. Such bacilK 

 did not produce death when inoculated into animals that 

 were ordinarily susceptible. Yet animals that were vaccinated 

 with this virus were able afterward to resist inoculation with 

 fully virulent anthrax bacilU. (See Bacillus anthracis, Part IV.) 



In the case of erysipelas of swine (French, rouget; German, 

 Schweinerothlauj) Pasteur secured bacilli of diminished viru- 

 lence by injecting viiTilent bacilli into relatively insusceptible 

 animals. The animal used was the rabbit. The bacilU were 

 passed through several rabbits in succession. Cultures taken 

 from the last of the series produced a milder form of the dis- 

 ease and an amount of immunity the value of which is in 

 dispute. 



In still another disease, black-leg of cattle or symptomatic 

 anthrax (French, charbon symptomalique; German, Rausch- 

 brand), an attenuated virus is secured by the use of heat. 

 The pulp from the infected muscle of a diseased animal, 



