194 PARASITISM 



Everyone is familiar with the ordinary facts of immunity from 

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

 tious 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, they 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 all 

 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 

 disease and (b) passive immunity by vaccination with organisms 

 which produce the disease or by their products. If organisms 



