CHAPTER VI 



IMMUNITY 



BY immunity is meant non-susceptibility to a given 

 disease or organism; thus a yellow fever immune 

 is a person who is not susceptible to yellow fever. 



Immunity may be natural or acquired. 



Natural Immunity. Man is susceptible to infec- 

 tion with many microorganisms, some of which are 

 harmless to the lower animals, and thus it is said that 

 these animals possess a natural immunity to these 

 infections. The bacillus of typhoid fever produces 

 serious disturbances in man, but rarely any in 

 cattle, and the cold-blooded animals are not suscep- 

 tible to tetanus, which in man and the higher ani- 

 mals is a serious infection. 



The physical differences existing between the 

 warm and cold blooded animals, the vertebrates 

 and invertebrates, and many other distinctions in 

 the various groups of animals, may all be taken into 

 account in the consideration of natural immunity. 



It was formerly held that certain races of men 



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