178 IMMUNITY 



warm-blood temperature, certain cold-blooded animals become sus- 

 ceptible to tetanus and other infections. Likewise, warm-blooded ani- 

 mals are immune to many bacteria because they cannot grow at the 

 temperature of the animal body. This is true of the greater number 

 of the saprophytic bacteria found in drinking-water. A bacterium 

 which cannot grow and multiply at the temperature of the animal body 

 cannot infect the animal. Such an organism may produce a substance 

 outside the body and at its growth temperature, which is poisonous 

 to a warm-blooded animal, but it cannot infect the animal. 



Like differences in susceptibility are observed in birds and mam- 

 mals. So far as we know, under natural conditions, there is no trans- 

 ference of infection between these classes. With larger doses, or 

 under unnatural conditions the infection of mammals may be trans- 

 ferred to birds. Chickens become susceptible to some of these infec- 

 tions, such as anthrax, when the body temperature is kept low. Chick- 

 ens have tuberculosis, but the bacillus of avian tuberculosis grows at 

 45 and even at 50 C. while that of the human does not grow above 

 41 C. (105.8 F.). While barnyard fowls have frequent opportunity 

 to pick up the sputum of tuberculous men, they do not become infected 

 in that way, nor are men infected with avian bacilli, although the 

 opportunities are quite frequent. It must not be inferred that tem- 

 perature is the only factor in these differences in susceptibility. It 

 seems to be one factor. The difference in metabolism is probably 

 quite as great a factor, possibly much greater, but its value is not so 

 easily determined. Here also, the poisons produced by avian infective 

 bacteria may prove harmful to mammals. Man may eat birds dead 

 of chicken cholera and by doing so he does not acquire the disease, 

 but he may be more or less severely poisoned by the bacterial products. 



Between herbivorous and carnivorous animals the differences are 

 not so great. The latter are, as a rule, much less susceptible but suc- 

 cumb to large inoculations. Anthrax, one of the most universal infec- 

 tions, is not known to appear in epidemic form among carnivorous 

 animals under natural conditions. There are traditions that this has 

 happened, but these are not based on reliable information. In his 

 susceptibility to infection man is more closely related to the herbivora 

 than to the carnivora. Beasts of prey may become infected with 



