74 INFECTIOUS AND PARASITIC DISEASES. 



animals. To account for it, there is obviously some 

 subtle difference in the vital processes of different 

 individuals, and in the same person at different times. 



Every healthy person is by nature endowed with the 

 means of combating disease, but his natural defenses, 

 while they may be strengthened, may also be weakened 

 by those forces and influences which surround him. 

 Nor before the tribunal, disease, are all men born equal, 

 since progenitors may transmit to their offspring con- 

 stitutions defective in defensive force. Furthermore, 

 during the constructive (infancy and childhood) and 

 degenerative (old age) periods of life the protective 

 forces are, on the other hand, weakened through imma- 

 turity, on the other, by the fact that they are declining 

 with age. Therefore, to environment, heredity, and 

 age, must we look for an explanation of any imperfec- 

 tion in our natural resistance to disease. 



Predisposition. The absence of resistance to 

 disease, i.e., susceptibility, has been given the compre- 

 hensive title predisposition, and we characterize as 

 predisposing every influence or cause which tends to 

 weaken the vital forces and therefore predisposes to 

 disease. In every infection such causes always play 

 an important role, usually more important than the 

 infectious agents, a fact to which too little heed is given 

 in the hurly-burly of life. The attitude of the world 

 in this respect is not unlike its greater faith in the cura- 

 tive powers of drugs, than in a disciplinary regulation 



