CHAPTER V. 



IMMUNITY. 



THAT certain races of animals and men, and certain 

 individuals among these, are more refractory to disease 

 than others, is a fact which has long been known. 

 Experience and observation have taught us, further, 

 that the same individuals are at one time more resistant 

 to disease than at another. This inborn or spontaneous 

 refractory condition is termed natural immunity, in con- 

 tradistinction to that acquired by recovery from disease. 



As in bacteria, we distinguish between the ability to 

 produce poison and the power to multiply in the body, 

 so here we may distinguish between immunity to poison 

 and immunity to the development of bacteria. 



With regard to variations in susceptibility, certain 

 known facts have been ascertained. Thus, cold-blooded 

 animals are generally insusceptible to infection from 

 those bacteria which produce disease in warm-blooded 

 animals, and vice versa. This is readily explained by 

 the inability of the bacteria which grow at the tem- 

 perature of warm-blooded animals to thrive at the 

 temperature existing in cold-blooded animals. But dif- 

 ferences are observed not only between warm-blooded 

 and cold-blooded animals, but also between the several 

 races of warm-blooded animals. The anthrax bacillus 

 is very infectious for the mouse and guinea-pig, while 

 the rat is not susceptible to it unless its body resistance 



