244 SUSCEPTIBILITY AND IMMUNITY. 



demic prevalence of this disease would show that second attacks are 

 more common than is indicated by these figures. 



That immunity may result from a comparatively mild attack as 

 well as from a severe one is a matter of common observation in the 

 case of small-pox, scarlet fever, yellow fever, etc. ; and since the dis- 

 covery of Jenner we have in vaccination a simple method of produc- 

 ing immunity in the first-mentioned disease. The acquired immunity 

 resulting from vaccination is not, however, as complete or as per- 

 manent as that which results from an attack of the disease. 



These general facts relating to acquired immunity from infectious 

 diseases constituted the principal portion of our knowledge with re- 

 ference to this important matter up to the time that Pasteur (1880) 

 demonstrated that in the disease of fowls known as chicken cholera, 

 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. 



