CONTAGIUM 101 



spores for many years means life or death to animals and 

 people. 



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 immunizing 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. 



