SUSCEPTIBILITY AND INFECTION 69 



gain entrance into the body of the host. A few virulent 

 germs may succeed in establishing an infection where a 

 larger number of attenuated ones might fail. 



The route by which the germ gains entrance to the 

 body has also an influence upon the susceptibility of the 

 animal to infection, intravenous injection of germs pro- 

 ducing infection where subcutaneous injections fail. 

 Certain germs have certain routes of entrance to the 

 body, and entrance by any other route reduces the 

 chances of infection or reduces the intensity of the pro- 

 cess when infection occurs. The resistance to cholera 

 infection is greater in subcutaneous infection than when 

 infection occurs by way of the intestinal canal, the 

 usual route. Lowered vitality from an attack of one 

 disease often increases the susceptibility to another in- 

 fection, as in the susceptibility to tuberculosis following 

 measles. 



Fatigue is, without doubt, responsible for a great deal 

 of increased susceptibility. White rats, which are im- 

 mune to anthrax, become susceptible when fatigued. 

 Starvation also reduces the resistance. Pigeons, which 

 are immune to anthrax, become susceptible upon starv- 

 ing. Heat, cold, and moisture also increase the chance 

 of infection, as was shown by Pasteur, who, by immers- 

 ing a hen in water, made her susceptible to anthrax, to 

 which chickens are normally immune. Improper food 

 and drugs may increase the chances of infection. Han- 

 kin fed immune rats on sour milk and bread and made 



