246 LOUIS PASTEUR. 



THE RETURN TO VIRULENCE. 



AFTER having reduced the microbes of fowl cholera 

 and splenic fever to all degrees of virulence, and 

 brought them to a point where they could no longer 

 multiply in the bodies of animals inoculated with 

 them, and fixed them in media appropriate to their 

 life, Pasteur asked himself whether it would not be 

 possible to restore to these attenuated microbes 

 weakened to such a degree as to have lost all viru- 

 lence a deadly virulence, and to render them again 

 capable of living and multiplying in the bodies of 

 animals. 



Experiment soon confirmed this mental previ- 

 sion. An attenuated splenic fever virus which could 

 cause no danger of disease or death to guinea-pigs of 

 a year, or a month, or even a week old, could kill a 

 little guinea-pig just born, or one or two days old. 

 The attenuated microbe could multiply itself in the 

 blood of so young an animal. We can well imagine 

 that in an animal, scarcely formed, the power of oxy- 

 genation of the blood globules is not as yet capable of 



