198 PARASITISM 



against disease, they do not form the sole means of protection. 

 The phenomena of immunity furnish another and more subtle 

 illustration of physiological adaptation. 



Everyone is familiar with the ordinary facts of immunity 

 from disease. In a community in which some contagious or in- 

 fectious disease is epidemic, some individuals do not acquire the 

 disease, if exposed to it. These individuals are said to be im- 

 mune, and of these there are usually two classes; in one class, the 

 individuals have never had the disease, but enjoy what is called 

 natural immunity. Again, other individuals take the disease but 

 have it in mild form; they are said to be slightly susceptible to 

 the disease, but have sufficient natural immunity to make it a 

 light case. Still others are highly susceptible, and succumb. 



In a similar way, entire races may be naturally immune to 

 diseases that are ordinarily fatal to other races. Thus the 

 horse and ass are highly susceptible to the organism of glanders, 

 but cattle, sheep and fowls can be injected with large doses of 

 the glanders organism without ill-effects they are naturally 

 immune. Many diseases of lower animals, such, for example, 

 as hog cholera, swine plague, chicken cholera, mouse septi- 

 caemia, etc., fatal to these animals, are harmless to human 

 beings. On the other hand, some diseases of man, like scarla- 

 tina, whooping cough, yellow fever, etc., are harmless to lower 

 animals, while some others like anthrax, tuberculosis, etc., are 

 equally dangerous both to man and lower animals. 



Most of us have passed through the ordinary diseases of 

 childhood ourselves whooping cough, chicken-pox, mumps, 

 and some through smallpox and scarlet fever, none of which we 

 expect to have a second time because of the acquired immunity 

 which these diseases have left in us. 



Again, most of us have been vaccinated against smallpox, and 

 some of us against typhoid fever, and neither of these diseases 

 may be expected after such vaccination, which has given us an 

 acquired immunity. This vaccination has produced changes in 

 the physiological mechanism similar to the changes produced 

 by the diseases themselves. Thus acquired immunity may be 

 of two types (a) active immunity, through experience of the dis- 



