TUBERCULOSIS 125 



between tuberculous infection and tuberculous disease. 

 Almost every child in civilized communities is exposed 

 to tubercle bacilli and infection takes place either 

 through the respiratory or digestive tract, so that by 

 fifteen years of age, nearly every child has become in- 

 fected. It is true that nearly all adults harbour tu- 

 bercle bacilli in their bodies, but only when for some 

 special reason these bacilli begin to grow or when 

 additional large numbers of bacilli enter the body does 

 the individual become affected with the disease tubercu- 

 losis. 



TUBERCULOSIS IN MAN AND ANIMALS. The natural 

 habitat of the tubercle bacilli is the bodies of men and 

 domestic animals. In the aboriginal Negro, tubercu- 

 losis is rarely found unless he is brought in contact with 

 civilization. Occasionally tuberculosis is found in wild 

 animals but usually when they have become domesti- 

 :ated. Three special types of tubercle bacilli are 

 known: the human, bovine, and avian. The human 

 strain is most virulent to man, the bovine less virulent 

 and the avian bacilli are a negligible factor in human 

 tuberculosis. We have already seen that tubercle in- 

 fection is almost ubiquitous among civilized people of 

 the human race. This is also true of the bovine races, 

 but in other animals it is far less common. The ba- 

 rilli, however, will live for months outside of the body 

 but are killed by sunlight. The disease tuberculosis, 

 although occurring in but a small proportion of those in- 

 fected, is one of the most serious of diseases. 



During the past century one seventh of all deaths 

 the result of tuberculosis. During the year 1921 



