DISCOVERY OF VACCINES 285 



would; it dies, or at least is very sick, the extent of the 

 disease and the danger of death being in inverse ratio 

 to the amount it had suffered in the previous inocula- 

 tion, for the slightest disease produced by inoculation 

 serves as some protection. 



Instead of making the three experiments which pre- 

 cede, we might, clearly, arrange a greater number 

 extending over the period of life of the virus, six, ten, 

 etc., in other words, interpose between the most sensi- 

 tive animal and the most resistant, a whole series of 

 animals differing in degree of immunity, each one of 

 which will acquire a degree of immunity corresponding 

 to the amount of vaccine which it can endure without 

 dying. The more severe the disease the greater the 

 protection the disease will afford it. 



All these animals, identical in appearance, different 

 in reality, clearly behave in a very different manner 

 toward the same microbe of virulent cholera. Some, 

 vaccinated, resist it without any trouble. Others, little 

 vaccinated or only recently, will become sick or die. 

 Furthermore, an animal, vaccinated or not, behaves 

 very differently toward microbes of unequal attenua- 

 tion. It is the variable result in this conflict which 

 makes the variety of pathological cases, and there are 

 not on the palette of any painter colors enough to des- 

 ignate the innumerable differences in receptivity which 

 we find in the virulent diseases. 



IV 

 ANTHRAX IS ALSO A VIRUS DISEASE 



As the study of chicken cholera progressed Pasteur 

 devoted much thought to anthrax on which he was work- 

 ing at the same time. He wished to study its etiology, to 



