308 PASTEUR: THE HISTORY OF A MIND 



According to our way of looking at things, nothing is less 

 surprising. The word ''virulence" sums up the result 

 of the conflict between two organisms. It is necessary, 

 therefore, to take into account the qualities of the two 

 adversaries. 



VIII 

 RETURN TO VIRULENCE 



We shall reach the same conclusion by an inverse 

 method, that is, by examining the conditions determin- 

 ing the return of virulence in a microbe which has lost it. 

 We know that these positive and negative variations of 

 virulence may be produced by simple changes in the 

 culture medium, but, from this standpoint, they are of 

 little interest. These variations become interesting only 

 as they manifest themselves in living creatures. Let us 

 see, therefore, if we cannot revive virulence by passage 

 through different species of animals unequally sensitive. 



We have obtained, it will be remembered, a strain of 

 the anthrax bacteridium absolutely harmless, then one 

 very much weakened, still able to kill guinea-pigs a day 

 old, but not to kill older guinea-pigs nor other species of 

 animals, then, starting from this one, a whole series of 

 microbes more and more virulent. Can one bring back 

 the most attenuated strains to a state of the highest 

 virulence? Experience replies, "No," in the case of a 

 completely non-virulent anthrax bacteridium, for it 

 escapes our experimentation by refusing to grow in any 

 living organism; it is henceforth fixed, and if it ever 

 returns to virulence, it will be by passing through a new 

 species of animal different from those which hitherto 

 have been shown to be capable of contracting anthrax. 



But the case is different for those strains which still 



