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 
