72 THE STORY OF THE BACTERIA. 



hypothesis around which we try to crystallize 

 our accumulating knowledge is that in heredi- 

 tary predisposition to tuberculosis this in- 

 herent resisting capacity of the cells to the 

 incursions of the germs is in some way dimin- 

 ished. This is very indefinite, it is true, but 

 it is the best we can do at present. We know 

 that tuberculosis is never caused by any other 

 thing than the tubercle bacillus, and that even 

 in persons predisposed by inheritance or other- 

 wise to the disease it cannot occur unless this 

 particular germ gets into the body from out- 

 side. The germ itself has rarely, if ever, been 

 shown to be directly inherited. If, therefore, 

 we could keep this particular germ away from 

 human beings, there would be no more tuber- 

 culosis, no matter what the inherent tendencies 

 of the individual might be. 



Let us now see what our knowledge teaches 

 us as to the sources of infection with the 

 tubercle bacillus. In the first place, we should 

 not forget that as the early stages of the dis- 

 ease are often very insidious in their progress, 

 and as an individual may actually have a 

 moderate degree of tubercular disease for a 



