96 ACTIVITIES OF BACTERIA. 



served, due to a local lowering of the bacteria-resisting property of the 

 inoculated animal at the point of the injection. 



III. Predisposition and Congenital Immunity 

 (Resistance). 



The susceptibility of various species and single individ- 

 uals to various infectious diseases from birth is striking 

 and not easily explained. 



In the first place, certain species of animals are naturally 

 absolutely immune l to certain infectious agents ; for 

 example, man to murrain, cattle to glanders, all experi- 

 mental animals to syphilis, malaria, and gonorrhea. 



Other diseases occur at least only rarely and with diffi- 

 culty in certain varieties of animals ; for example, 

 anthrax in certain races of pigeons, rats, and sheep. 

 Here there exists a relative immunity. Relative immu- 

 nity is usually proportionate to the strength and maturity 

 of the animal. All kinds. of injury (hunger, cold, over- 

 exertion, incorporation of certain poisons) lessen the im- 

 munity, — considerably increase the disposition, — so that 

 a large number of animals, weakened in this way, succumb 

 to a subsequent infection. 



It is, therefore, necessary, with every newly isolated 

 variety of bacterium of which one desires to ascertain the 

 pathogenic action, to include in the test the most varied 

 animals, if the first chosen animal gives a negative result. 

 Our principal experimental animals are : White house 

 mice, white rats, guinea-pigs, rabbits, chickens, pigeons, 

 and, for special purposes, monkeys. More rarely we em- 

 ploy gray house mice, rats, field mice, gophers, dogs, cats, 

 cows, sheep, pigs, and horses. The most desirable ex- 

 perimental animal is the guinea-pig, but it requires good 

 care. It is recommended by its convenient size, docility, 

 and limited consumption of food. Animal diseases are 

 usually much easier to investigate and explain absolutely 

 than human diseases, because the susceptible experi- 



1 It is especially noteworthy that very closely related varieties often 

 conduct themselves very differently in this regard ; as, for example, 

 the glanders bacillus easily infects field mice, but not house mice ; 

 anthrax bacillus kills house mice with almost absolute certainty, but 

 is not pathogenic for the rat, etc. 



