THE CAUSE OF INFECTIOUS DISEASE. 243 



possesses within his organism, that is in his or- 

 gans, tissues, cells and body-fluids, at a given 

 time and place, a definite kind and amount of 

 potential energy or cause. This, when it be- 

 comes manifest, we designate variously as a phy- 

 siological property or irritability, as a morbid 

 susceptibility, as a predisposition to disease, or 

 as immunity. A predisposition to certain dis- 

 eases exists among different races and species. 

 Negroes for example are infected with small- 

 pox much more easily than Europeans, while 

 the latter not only sicken more easily with yel- 

 low-fever but the disease assumes in them a 

 more fatal character. Variation in suscepti- 

 bility to disease is also found among indivi- 

 duals of the same species, as is established 

 by such a fact as that among our popula- 

 tion only 3 to 7 per cent, contract cholera, 

 while others of the population, although un- 

 der the same conditions, resist the disease. 

 In spite of the ample opportunities of infec- 

 tion with tuberculosis which are afforded 

 every one, only some 20 to 25 per cent, of the 

 population really contract any form of the 

 malady. Indeed, if we consider the most dreaded 

 of all known infections, the Black Death of 

 the Middle Ages, we are told that only one 

 fourth of the whole population of Europe con- 

 tracted the disease, some 75 per cent., therefore, 



