122 VETERINARY BACTERIOLOGY 



Variation of Individuals in Susceptibility to Disease. Pre- 

 disposing Factors. The same individual varies greatly at times 

 in his ability to resist infection by bacteria. Age frequently deter- 

 mines resistance to certain infections. For example, there are 

 diseases, such as diphtheria and whooping-cough, which are 

 much more common among children than among adults. Black- 

 leg rarely attacks adult cattle. Advancing age seems to bring 

 increased resistance to such infections. The mechanism of this 

 kind of resistance to infection is not well understood. Hunger 

 and thirst reduce the resistance of the body and predispose to 

 infection. Exposure to excessive heat or the chilling of the body 

 surfaces by cold will also reduce the body resistance so that organ- 

 isms that ordinarily cannot produce disease gain a foothold. 

 Fatigue has been demonstrated experimentally to render animals 

 and man more susceptible to infection. The classic example of 

 this diminution of resistance by fatigue is that of the white rat, 

 which is normally immune to anthrax, but which, when exhausted 

 by work in a treadmill, becomes susceptible to the disease and will 

 succumb to infection. 



Types of Immunity. Immunity may be divided into two types, 

 natural and acquired. The former may be subdivided again into 

 racial or specific immunity and individual immunity. Acquired 

 immunity may be either active or passive. 



Natural immunity is congenital, that is, it is not acquired after 

 birth. A racial immunity is one possessed by all the members of 

 a group of individuals. Disease frequently cannot be transmitted 

 from one species of animal to another, for example, man does not 

 acquire many of the diseases of animals, such as hog-cholera and 

 dog distemper, and, on the other hand, many diseases of man, such 

 as measles, whooping-cough, and typhoid fever, cannot be trans- 

 mitted to the lower animals. It is also said that the Algerian alone 

 among the breeds of sheep is naturally immune to anthrax. In 

 New York City it has been found that the Russian and Polish 

 Jews are much more resistant to tuberculosis than are certain other 

 races, particularly the Irish and the negroes at least the death- 

 rate among the former due to this disease is much lower. Individ- 

 uals are also found who are naturally immune to disease. This re- 

 sistance is perhaps more seeming than real in many cases, and has 



