286 LOUIS PASTEUR. 



microbe is virulent in an animal, when it has the 

 power of swarming in the body of that animal, after 

 the manner of a parasite, and of producing, by the re- 

 newal of its own life, disturbances which cause disease 

 and death. If this microbe has lived in any species 

 of animal that is to say, if several times over it has 

 passed from the body of one individual into that of 

 another of the same kind, without having been sub- 

 jected to any sensible exterior influence during its 

 passage we may consider that the virulence of this 

 parasite has reached a fixed and maximum state for 

 the individuals of that race. The splenic fever para- 

 site pertaining to sheep, for instance, varies little 

 from one subject to another or from one year to 

 another in the same country ; this must be attributed, 

 doubtless, to the fact that, in its successive passages 

 through the sheep, the habit of the parasite to live in 

 sheep has, so to speak, attained a definite state. It is 

 thus with the virus of the Jennerian vaccination. But 

 the virulence of a virus which is not at its maximum 

 may be essentially modified by its passage into a 

 succession of individuals of the same race. It will be 

 remembered how, when Pasteur and his assistants 

 wished to increase progressively the virulence of the 

 virus of chicken cholera and splenic fever, so as to 

 bring them at last to their maximum intensity, these 

 viruses were first inoculated into young subjects, and 

 from them successively into older ones. 



