III. 



SUSCEPTIBILITY AND IMMUNITY. 



No questions in general biology are more interesting, or more 

 important from a practical point of view, than those which relate to 

 the susceptibility of certain animals to the pathogenic action of cer- 

 tain species of bacteria, and the immunity, natural or acquired, from 

 such pathogenic action which is possessed by other animals. It has 

 long been known that certain infectious diseases, now demonstrated 

 to be of bacterial origin, prevail only or principally among animals 

 of a single species. Thus typhoid fever, cholera, and relapsing 

 fever are diseases of man, and the lower animals do not suffer from 

 them when they are prevailing as an epidemic. On the other hand, 

 man has a natural immunity from many of the infectious diseases of 

 the lower animals, and diseases of this class which prevail among 

 animals are frequently limited to a single species. Again, several 

 species, including man, may be susceptible to a disease, while other 

 animals have a natural immunity from it. Thus tuberculosis is 

 common to man, to cattle, to afpes, and to the small herbivorous ani- 

 mals, while the carnivora are, as a rule, immune ; anthrax may be 

 communicated by inoculation to man, to cattle, to sheep, to guinea- 

 pigs, rabbits, and mice, but the rat, the dog, carnivorous animals, and 

 birds are generally immune ; glanders, which is essentially a disease 

 of the equine genus, may be communicated to man, to the guinea- 

 pig, and to field mice, while house mice, rabbits, cattle, and swine 

 are to a great extent immune. 



In addition to this general race immunity or susceptibility we 

 have individual differences in susceptibility or resistance to the ac- 

 tion of pathogenic bacteria, which may be either natural or acquired. 

 As a rule, young animals are more susceptible than older ones. 

 Thus in man the young are especially susceptible to scarlet fever, 

 whooping cough, and other "children's diseases," and after forty 

 years of age the susceptibility to tubercular infection is very much 

 diminished. Among the lower animals it is a matter of common 

 laboratory experience that the very young of a susceptible species 

 may be infected when inoculated with an "attenuated culture" 

 which older animals of the same species are able to resist. 



