CHAPTER IV 



SUSCEPTIBILITY TO INFECTION 



TT has long been known that certain infectious dis- 

 eases prevail only or principally among animals 

 of a single species, while others are communicable to 

 several species, including man himself. Thus typhoid 

 fever, cholera, and relapsing fever are diseases of 

 man, and during their epidemic prevalence none of 

 the domestic animals contracts any 'of these diseases. 

 On the other hand, the lower animals are subject 

 to various infectious diseases which may prevail as 

 fatal epidemics but which are never communicated 

 to man for example, chicken cholera, hog cholera, 

 swine plague, rinderpest, foot-and-mouth disease. 

 Again, several species, including man, may be sus- 

 ceptible to a disease while other animals have a 

 natural immunity to it. Thus tuberculosis is com- 

 mon to man, to cattle, to apes, and to the small 

 herbivorous animals (by inoculation), while the carni- 

 vora, as a rule, are immune ; anthrax may be 



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