THE ATTENUATED VIEUS, OR VACCINATION. 221 



ever been a discovery more mysterious in its causes 

 and origin, standing, as it does, alone in the history 

 of medicine, and for more than a century defying all 

 comparison ? 



After dwelling long on Jenner's discovery this ques- 

 tion arose in Pasteur's mind : If contagious maladies 

 do not repeat themselves, why should there not be 

 found for each of them a disease different from them, 

 but having some likeness to them, which, acting 

 upon them as cow-pox does upon small-pox, would 

 have the virtue of a prophylactic ? A chance occur- 

 rence, one of those chances which not unfrequently 

 occur to those who are steadfastly looking out for 

 them, opened out to Pasteur the way to a discovery 

 which may well be called one of the greatest discoveries 

 of the age. 



In causing the microbe of fowl cholera to pass from 

 culture to culture, in an artificial medium, a sufficient 

 number of times to render it impossible that the least 

 trace of the ..virulent matter from which it originally 

 started should still exist in the last cultivation, 

 Pasteur gave in an absolute manner the proof that 

 infectious microbes are the sole authors of the 

 diseases which correspond to them. This culture may 

 be repeated ten, twenty, a hundred, even a thousand 

 times : in the latest culture the virulence is not ex- 

 tinguished, or even sensibly weakened. But it is a 

 fact worthy of attention that the preservation of the 



