INFECTION AND IMMUNITY 357 



can be determined, certain animals are exempt from 

 the injurious effects of poisons or from invasion by cer- 

 tain microparasites. 



Remembering the suggestions concerning the habit- 

 uation of animals to poisonous foods and the immu- 

 nity they seem to enjoy in consequence, we turn to an- 

 other aspect of the problem. 



Are microorganisms subject to the same ill effects 

 experienced by the higher organisms when the quality 

 of their nourishment is altered? Can this be the expla- 

 nation of the rapidity with which they invade the body 

 of one host and die out in that of another? It seems 

 justifiable to apply the same method to both, and by 

 doing so their behavior under certain conditions will be 

 explained. 



It is much more satisfactory to consider the subjects 

 immunity and infection with reference to the host, 

 though to lose sight of the microparasite and of the recip- 

 rocal relations of host and parasite will be to lose much 

 that is of fundamental importance. Indeed, almost 

 everything that is said of one applies with equal 

 force to the other. 



Experiments with the bacteria have shown that they 

 quickly accustom themselves to certain hosts. Thus, 

 streptococci by special manipulations may be made so 

 virulent for rabbits that a mathematical calculation 

 may show that a single coccus may be fatal. At the 

 same time, however, they may not increase in virulence 

 for other kinds of animals. Not only do they become 

 habituated to a certain kind of animal, but there is 

 evidence to show that they may even become habituated 

 to some particular organ of the animal; thus, streptococci 

 taken from a lesion of the kidney of an animal and in- 

 jected into the circulation of a new animal of the same 

 kind are said to colonize in greater numbers in the 

 kidneys than elsewhere in the body. 



Here we seem to have definite, though not necessarily 

 fixed results following habituation. 



