CHAPTER XI 



IMMUNITY 



It is well known that certain infectious diseases occur 

 naturally only among some of the lower animals, and 

 do not affect man; while, conversely, others appear to 

 attack only man. Among the latter may be mentioned 

 typhoid fever, syphihs, gonorrhea. In speaking of the 

 resistance evidently possessed by certain individuals or 

 certain species, we make use of the term natural iwr- 

 munity. Thus, chickens and frogs possess a natural 

 immunity against tetanus (lock-jaw); dogs, a natural 

 immunity against anthrax; goats, a natural immunity 

 against tul^erculosis, and man, a natural immunity 

 against certain diseases of cattle. This natural im- 

 munity, however, is not absolute. Chickens, for example, 

 can be infected with tetanus if the body is chilled, and 

 frogs can be made susceptible to tetanus by keeping 

 them unduly warm. 



Another form of immunity is that observed in indi- 

 viduals who have had one attack of a particular infec- 

 tion; thereafter they are practically safe from a second 

 attack. These individuals are said to possess an acquired 

 immunity. This form of immunity is well illustrated in 

 scarlet fever, measles, small-pox, yellow fever. Often 

 this immunity lasts throughout the lifetime of the indi- 

 vidual, though there are exceptions. 



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