6 INFECTION AND IMMUNITY. 



be obtained before it could be safely injected into 

 animals for the purpose of producing immunity, 

 for if the unaltered virus were injected the viru- 

 i en t infection would result. Accordingly, Tous- 

 saint heated the blood of a sheep which had died 

 of anthrax, to a temperature of 55 C. for ten 

 minutes, then injected it into a number of sheep. 

 Some of the animals died of anthrax, while others 

 suffered only a mild attack from which they re- 

 covered; the latter were found to be immune to 

 a subsequent inoculation with virulent blood. In- 

 asmuch, however, as some of Toussaint's animals 

 had died of anthrax, Pasteur concluded that there 

 was some grave error in technic. He considered 

 that Toussaint's method probably killed or atten- 

 uated the fully-developed bacilli, but did not in- 

 jure the spores of the parasite (Koch had pre- 

 viously shown the existence of anthrax spores). 

 After much experimentation;, Pasteur hit on the 

 plan of growing the bacillus at a temperature of 

 42 C., obtaining in this way a culture of the 

 fully developed organism which had a low viru- 

 lence, but which did not form the dangerous 

 spores. When sheep were inoculated with the 

 proper amount of this culture, which became 

 known as anthrax vaccine, they had a mild attack 

 of the disease, which rendered them immune to 

 virulent inoculations. 



Hydro- With the possibility of protective inoculation 

 phobia. w ^k a i mown virus actually demonstrated, sim- 

 ilar procedures were tried with other animal dis- 

 eases of known bacterial etiology, with the result 

 that successful vaccines against chicken cholera 

 and swine plague were developed. Somewhat 

 later, having failed in their attempts to discover 



