CHAPTER XIX. 



IMMUNITY. 



Introductory. By immunity is meant non-susceptibility to 

 a given disease or to a given organism, either under natural 

 conditions or under conditions experimentally produced. 

 The term is also used in relation to the toxines of an organ- 

 ism. 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 passing through an 

 attack of the disease, or by artificial means of inoculation. 

 It has been shown 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 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 artificially be made to infect guinea-pigs, 

 though they do not do so naturally. Immunity may thus 

 be of very varying degrees, and accordingly the use of the 

 term has a correspondingly relative significance. Such a 

 thing as absolute immunity is scarcely known, just as we 



