IMMUNITY. 595 



tion of this nature takes place with greater vigor 

 and rapidity in a strong, healthy person than in 

 one of lower vitality. Aside from the question of 

 individual resistance, recovery or progressive infec- 

 tion may depend on the smaller or larger amount 

 of bacilli which gained entrance to the body, as 

 well as on their virulence. Experiments show that 

 susceptible animals recover from minute doses, 

 whereas they succumb to somewhat larger doses of 

 bacilli. 



Various external influences increase susceptibil- Predisposing: 

 ity and resistance. Tuberculosis is to no small de- 

 gree a disease of the poor, who so frequently live 

 in an undernourished condition, in crowded, dirty 

 rooms, with little sunlight and fresh air. The 

 disease is more common in the city than in the 

 country, where an outdoor life is the rule. Alco- 

 holism, diabetes, measles, scarlatina, whooping 

 cough often, and influenza not infrequently, are 

 precursors of tuberculosis. Conditions which favor 

 anemia, as pulmonary stenosis (rare), predispose 

 to pulmonary tuberculosis, whereas insufficiency of 

 the left heart, accompanied by congestion of the 

 lungs, is not often associated with the disease, al- 

 though it has no influence in preventing infection 

 in other organs. Tuberculosis is more frequent 

 during the first two or three years of life, when 

 children are so commonly confined, than from the 

 third to the fifteenth year, when they live in the 

 open air so largely. From the fifteenth year to 

 middle life or later the disease increases in fre- 

 quency because of greater exposure to infection. 

 Physicians who are familiar with tuberculosis in 

 Scandinavian countries and in America comment 



