220 LOUIS PASTEUR. 



THE ATTENUATED VIRUS, OR VACCINA- 

 TION OF VIRULENT DISEASES. 



THE VACCINE OF FOWL CHOLERA. 



AMONG the scourges which afflict humanity there are 

 none greater than virulent diseases. Measles, scar- 

 latina, diphtheria, small-pox, syphilis, splenic fever, 

 yellow fever, camp typhus, the plague of the East 

 what a terrible enumeration ! I pass over some, 

 such as glanders, leprosy, and hydrophobia. The 

 history of these diseases presents extraordinary cir- 

 cumstances. The most strange, assuredly, is that 

 which has been from all time established with a great 

 number of these diseases, that they are non-recurrent. 

 As a general rule, notwithstanding some rare excep- 

 tions, man can only have measles, scarlet fever, 

 plague, yellow fever once. What explanation, even 

 hypothetically, can be given of such a fact? Still 

 more difficult is it to explain how vaccination, which is 

 itself a virulent though benign disease, preserves from 

 a more serious malady, the small-pox? Has there 



