PRINCIPLES OF VETERINARY SURGERY 153 



b. Natural — i. e., as the result of an infectious 

 disease. Active only. 



Two principal types of immunity occur, natural and ac- 

 quired. Immunity may be acquired naturally as the result 

 of an infection, but this is a very different thing from 

 natural immunity. Immunity which is acquired artificially 

 may be active, as in vaccination; or passive, as in anti- 

 toxin injection. Furthermore, depending upon the nature 

 of the immune serum, whether its immunizing substance 

 neutralizes toxin or kills bacteria, immunity is said to be 

 antitoxic or antibacterial. 



Natural immunity is one which for the most part is in- 

 herited. This is usually the case where a whole class of ani- 

 mals is involved. On the other hand, the immunity may run 

 in certain breeds or families. The Poland China hog, for 

 example, is more susceptible to cholera than the "razor 

 back." Even in a given family there may be marked varia- 

 tions. An infection of distemper may attack six puppies, 

 while four may escape, and all these animals living under 

 the same conditions. In immunity, as in everything else, 

 the "personal equation" is an important factor. 



Old animals are by nature less liable to infectious dis- 

 eases than young. Young rats are markedly susceptible 

 to anthrax, while under ordinary conditions, old rats will 

 not succumb. In the human, measles, whooping-cough and 

 scarlet fever are practically limited to children; adults sel- 

 dom become infected. 



Hogs which overcome an attack of cholera may trans- 

 mit immunity to their offspring; on the other hand, the off- 

 spring of tubercular cattle may be immune or markedly sus- 

 ceptible to tuberculosis. Thus, susceptibility to infection 

 as well as immunity, may be inherited by a whole species 

 of animals. 



