CONTAGIUM 101 



spores for many years means life or death to animals and 

 lK'0])le. 



Susceptibility and immunity are only relative terms, and they 

 vary within wide limits. They depend on many factors : the 

 species of animals, age, exposure, fatigue, previous disease, 

 heredity, etc. An animal naturally immune to a certain germ 

 may contract infection when greatly fatigued. 



Immunity may be natural or acquired. Natural immunity 

 may be racial or individual ; to illustrate, the human resists 

 hog cholera, and the hog resists measles and smallpox. Indi- 

 viduals of the same species may differ widely in power of resis- 

 tance. In an outbreak of cholera in previously unexposed herds, 

 there are usually individuals that never miss a feed. Acquired 

 immunity is passive where one animal receives its immunizing 

 substances from another, as in serum — only treatment for hog 

 cholera. It is active when an animal is exposed to living virus, 

 either by natural or artificial process, and survives; thereafter 

 providing its own inununizing substance. Passive immunity is 

 temporary; active immunity is relatively permanent, as in the 

 serum-virus treatment for hog cholera or reduced virulence, 

 virus is used to vaccinate calves against blackleg. 



