

IMMUNITY AND SUSCEPTIBILITY. 93 



Natural Immunity. 



Natural immunity is the natural, inherited resisting 

 power peculiar to certain species of animals. 



A few diseases are so widely distributed as to take in all 

 of the orders of the animal kingdom ; thus, tuberculosis 

 has been observed in mammals, birds, reptiles, batrachi- 

 ans, and fishes. Nearly all diseases are, however, more or 

 less restricted, and are observed chiefly in animals with 

 certain common peculiarities. An example of this is 

 anthrax, which is a disease of warm-blooded animals, 

 and may infect the great majority of mammals and a 

 few birds, but will not infect the cold-blooded animals. 

 Very commonly among closely related animals great dif- 

 ferences in susceptibility exist, so that it can readily be 

 determined that anthrax is far more infectious for her- 

 bivorous than for carnivorous animals. Not infrequently 

 remarkable variations are observed in animals of the same 

 order; thus, among the rodents we find that while the 

 mouse, guinea-pig, and rabbit readily succumb to an- 

 thrax, the rat is immune. Rarely we find that differ- 

 ences of susceptibility exist among families and genera, 

 and even among species and varieties ; thus, the white 

 mouse is immune to glanders, the house mouse is not 

 very susceptible, but the field mouse is perhaps the most 

 susceptible of all animals. 



These differences also exist between the susceptibility 

 of man and the lower animals ; thus, while man, in com- 

 mon with the lower animals, suffers from anthrax, 

 glanders, actinomycosis, etc., he also suffers from cholera, 

 typhoid fever, syphilis, lepra, scarlatina, and a variety of 

 other diseases, never observed to occur spontaneously in 

 the lower animals, which in turn are frequently afflicted 

 with such diseases as symptomatic anthrax, hog cholera, 

 swine plague, chicken cholera, mouse septicemia, rabbit 

 septicemia, etc., which do not affect man. 



Racial differences of susceptibility also occur among 

 men ; thus, negroes are said to be immune to yellow fever, 

 and the Japanese are said to be immune to scarlatina, 



