SAFEGUARDS OF THE BODY 133 



little hidden arena. A new environment is estab- 

 lished both for the body cells and for the bacteria ; and 

 what we dramatize as a battle is really only the at- 

 tempt of each to adapt itself to the new conditions 

 furnished by the other. The one which adapts itself 

 most readily and completely and quickly wins, by 

 survival. 



Infectious diseases, then, are those which are in- 

 duced by the entrance into the body and the multipli- 

 cation there of disease-inducing micro-organisms. 

 These are most frequently bacteria ; but other lowly 

 beings, such as yeasts and minute animals called pro- 

 tozoa, are sometimes to blame. Each of these infec- 

 tious diseases has its peculiar characteristics by which 

 physicians recognize it. These features are especially 

 dependent upon the nature of the bacteria which in- 

 duce them : their ways of growing, the nature of the 

 poisons. which they set free, their tenacity of life, etc. 

 But the body cells have their particular vulnerabilities 

 to bacterial poisons, so that in one case it is the nervous 

 system, in another the lungs, in another the digestive 

 apparatus, which especially suffers. Moreover, as one 

 rose is redder than another, or one aromatic plant 

 more pungent than its fellow, so in one case the bac- 

 teria which gain access to the body may evolve a 

 more potent poison than in another, and then the dis- 

 ease may be of a more virulent type. So also an 

 individual may at the time of infection be much more 

 susceptible to the ravages of the germ than is usual, 

 and thus the victim of a graver form of disease. 



