IMMUNITY AND SUSCEPTIBILITY. 59 



extraordinary is the fact that a few drops of blood from 

 the recovered mouse injected into another will protect it 

 from tetanus. 



Immunity is the condition in which the body of an 

 animal resists the entrance of disease-producing germs, 

 or, having been compelled to allow them to enter, resists 

 their growth and pathogenesis. The resistance so mani- 

 fested is a distinct, potential vital phenomenon. 



Susceptibility is the opposite condition, in which, in- 

 stead of resistance, there is a passive inertia which allows 

 the disease-producing organisms to develop without oppo- 

 sition. Susceptibility is accordingly the absence of im- 

 munity. 



Immunity is either natural or acquired. 



Natural Immunity. By this term is meant the natural 

 and constant resistance which certain healthy animals 

 exhibit toward certain diseases. 



The white rat is peculiar in resisting anthrax. It is 

 almost impossible to develop anthrax in a healthy white 

 rat, but Roger found that such an animal would easily 

 succumb to the disease if compelled to turn a revolving 

 wheel until exhausted. Susceptibility which follows such 

 an exhaustion of the vital powers cannot be regarded as 

 other than accidental, and makes no exception to the 

 statement that the white rat is immune to anthrax. 

 Animals such as man, sheep, cows, rabbits, and white 

 mice are susceptible to anthrax, while birds and reptiles 

 are generally immune. The great difference in the morph- 

 ology between mammals and birds and reptiles, together 

 with the fact that their temperature, blood, and tissues 

 all differ, makes this immunity reasonably intelligible. 

 Morphological differences, however, will not suffice to 

 explain all cases, for the Caucasian nearly always suc- 

 cumbs to yellow fever, while the negro is rarely affected ; 

 and scarlatina, which is one of our commonest and most 

 dangerous diseases of childhood, is said to be unknown 

 among the Japanese. Nor is this all, for, close as is their 

 resemblance in all respects except color, the house-mouse, 



