CHAPTER XII 

 IMMUNITY 



FOR many years it has been observed that one attack of most 

 of the infectious diseases will protect an individual against subse- 

 quent attack, or at least future attacks will be of a less severe nature. 

 Crude attempts were frequently made by primitive people to ob- 

 tain this protection. Thus South African tribes tried to defend 

 themselves against snake bites by using a mixture of snake venom 

 and gum ; in the East in order to obtain protection against a severe 

 attack of smallpox people deliberately placed themselves in con- 

 tact with a mild case, hoping to contract the disease in a similar 

 form and thus obtain protection against a disfiguring and probably 

 fatal form. 



The object of these procedures was to obtain resistance or 

 immunity. The term immunity in its broadest sense may be ap- 

 plied to resistance in general. Ordinarily, however, it is applied 

 to the power of resisting disease which certain forms of life possess. 

 Health is a condition of immunity. So long as we are alive and 

 our body cells can continue to manufacture their specific protective 

 substances the bacteria on our skin and in our intestinal tracts 

 can do no harm, but the moment we die they penetrate our tissues 

 and disintegrate them. 



Immunity is of course the contrary condition to susceptibility. It 

 is not confined to the animal kingdom alone ; it occurs also among 

 plants. The theory of Welch attributes its possession even to 

 bacteria ; thus man is susceptible to the typhoid bacillus because 

 the typhoid bacillus is immune to man ; conversely man is immune 

 to the hay bacillus because the hay bacillus is susceptible to man. 



Pasteur tried to explain the production of immunity by his 

 " exhaustion theory," a theory now entirely disproved. He be- 



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