CHAPTER XIX. 



IMMUNITY. 



Introductory. By immunity is meant non-susceptibility to 

 a given disease or to a given organism under certain con- 

 ditions, and these conditions may either be such as occur 

 naturally, or may be experimentally produced. The term 

 is also used in relation to the toxines of an organism. 

 Immunity may be possessed by an animal naturally, and 

 is then usually called natural immunity, or it may be 

 acquired by an animal either by its passing through an 

 attack of the disease occurring under ordinary conditions, 

 or by artificial means of inoculation. We find, for example, 

 that certain diseases affect the lower animals but never 

 occur in the human subject, e.g., swine plague ; and, on 

 the other hand, diseases such as typhoid fever and cholera, 

 which are common in the case of the human subject, do 

 not under natural conditions affect any of the lower animals, 

 so far as is known. That is to say, man and the lower 

 animals respectively enjoy immunity against certain diseases, 

 when exposed to infection under ordinary conditions. 

 From this fact, however, it does not follow that when the 

 organisms of the respective diseases are introduced into 

 the body by artificial methods of inoculation, pathological 

 effects will not follow. We have seen above, for example, 

 that the organisms of cholera and typhoid may be made 

 to infect guinea-pigs artificially, though they do not do so 

 under natural conditions. Immunity may thus be of very 



