IMMUNITY. 199 



bodies, antitoxins proper, antiagglutinins, antilysins and anti- 

 precipitins. 



Resistance to Infection. Immunity from infectious dis- 

 eases is either natural or acquired, active or passive. 



By natural immunity is meant the inherited power possessed 

 by certain races or individuals, independently of size, habits 

 or surroundings, to resist infection to which other races or in- 

 dividuals are subject. This is illustrated by many examples. 

 Rats are ordinarily insusceptible to anthrax, whereas mice, 

 guinea-pigs, sheep, cattle in short, most animals are very 

 susceptible. Mice are not susceptible to diphtheria poison on 

 inoculation, while horses, sheep, goats, guinea-pigs and many 

 other animals are very susceptible. Even very nearly related 

 species or varieties often show difference in susceptibility. 

 House-mice are susceptible to mouse septicemia, European 

 field-mice are not. On the other hand, with glanders, house- 

 mice and white mice are less susceptible than European field- 

 mice are to this disease. Negroes are insusceptible to certain 

 diseases to which the white race is very subject, and also the 

 reverse. 



Instances of individual immunity are seen in every epidemic, 

 where persons escape when they are under the same conditions 

 as those who have contracted the disease. Instances also occur 

 in which nurses and others thrown with cases of highly infec- 

 tious diseases escape. Some of these cases,- it is true, belong 

 more properly to the category of immunity acquired by recovery 

 from an attack, since nurses and others thrown in contact with 

 an infectious case may suffer a very mild attack or even, prob- 

 ably become immunized without showing any symptoms of 

 disease. Nurses and physicians have been found with diph- 

 theria bacilli in their throats and yet not showing any symptoms. 

 Of course, these persons may have had natural immunity, but 

 it is equally possible that they may have become gradually 

 immunized. 



