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. 
