THE CAUSE OF INFECTIOUS DISEASE. 249 



social mal-adjustments, by improvement of the 

 locality in which we dwell, by changes in meta- 

 bolism through the introduction of better nu- 

 triment, or by regulation of the temperature 

 conditions of the body. Finally, we are able to 

 make use of the fact that by undergoing a 

 disease the predisposition to that disease is 

 removed and converted into its opposite, im- 

 munity. 



In no case can anything appear in the form 

 of disease which was not previously present in 

 the body as a predisposition ; external forces 

 are able merely to make this predisposition 

 apparent. It is therefore at the outset im- 

 portant to hold fast to the fact that we are in 

 a position to act upon a given physiological 

 organization by a whole series of changes in 

 external conditions ; we can either heighten a 

 predisposition or remove it. Herein lies also 

 the reconciliation of the physician's art, which 

 has reference to the individual, with the official 

 health regulations, which have regard to the 

 conditions making for the betterment of all. 

 When the physician, by thorough observation 

 and investigation, knows the conditions that 

 influence a given disposition in a definite way, 

 when he is scientifically trained and has a true 

 conception of hygiene, and is at once physician 

 and naturalist, then he is able to cure disease 



