SUSCEPTIBILITY AND IMMUNITY. 233 



which he had proved to be due to a specific microorganism, a mild 

 attack followed by immunity may be induced by inoculation with an 

 " attenuated virus " i.e., by inoculation with a culture of the patho- 

 genic microorganism the virulence of which had been so modified 

 that it gave rise to a comparatively mild attack of the disease in 

 question. Pasteur's original method of obtaining an attenuated virus 

 consisted in exposing his cultures for a considerable time to the ac- 

 tion of atmospheric oxygen. It has since been ascertained that the 

 same result is obtained with greater certainty by exposing cultures 

 for a given time to a temperature slightly below that which would 

 destroy the vitality of the pathogenic microorganism, and also by ex- 

 posure to the action of certain chemical agents (see Part Second, p. 

 124). 



Pasteur at once comprehended the importance of his discovery, 

 and inferred that what was true, of one infectious germ disease was 

 likely to be true of others. Subsequent researches, by this savant 

 and by other bacteriologists, have justified this anticipation, and the 

 demonstration has already been made for a considerable number of 

 similar diseases anthrax, symptomatic anthrax, rouget. 



A virus which has been attenuated artificially by heat, for ex- 

 ample may be cultivated through successive generations without re- 

 gaining its original virulence. As this virulence depends, to a con- 

 siderable extent at least, upon the formation of toxic products during 

 the development of the pathogenic microorganism, we naturally infer 

 that diminished virulence is due to a diminished production of these 

 toxic substances. 



There is reason to believe that a natural attenuation of virulence 

 may occur in pathogenic bacteria which are able to lead a sapro- 

 phytic existence during their multiplication external to the bodies of 

 living animals, and the comparatively mild character of some epi- 

 demics is probably due to this fact. 



Again, cultivation within the body of a living animal may, in 

 certain cases, cause a diminution in the virulence of a pathogenic 

 microorganism. Thus Pasteur and Thuiller have shown that the 

 microbe of rouget when inoculated into a rabbit kills the animal, but 

 that its pathogenic virulence is nevertheless so modified that a cul- 

 ture made from the blood of a rabbit killed by it is a suitable ' ' vac- 

 cine " for the pig. 



On the other hand, we have experimental evidence that the viru- 

 lence of attenuated cultures may be reestablished by passing them 

 through the bodies of susceptible animals. Thus a culture of the 

 bacillus of rouget, attenuated by having been passed through the 

 body of a rabbit, is restored to its original virulence by passing it 

 through the bodies of pigeons. And a culture of the anthrax bacillus 



