CHAPTER XI 



NATURAL IMMUNITY 



IT is hardly necessary to explain that absence of 

 susceptibility to an infectious disease constitutes 

 what is known as immunity for or against the disease 

 in question. Now this immunity may be natural or 

 acquired that is, due to inheritance or developed in 

 a susceptible individual subsequent to birth. We 

 have said in a preceding chapter that man is immune 

 as regards certain infectious diseases of the lower ani- 

 mals, and that many of the infectious diseases to 

 which he is subject are not transmitted to the domes- 

 tic animals with which he is most closely associated. 

 This natural immunity is not, however, in all cases 

 absolute and complete. For example, the white rat 

 possesses a remarkable immunity against anthrax, a 

 disease which may be communicated by inoculation 

 to sheep, cattle, rabbits, guinea-pigs, mice, and to man 

 himself. But it has been shown that this natural im- 

 munity of the white rat may be overcome by giving 



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