274. PARASITIC BACTERIA 
highly developed, while others yield easily to invasion. This 
we speak of as individual resistance. The resistance varies also 
with the vigor of the germs. Some epidemics of measles, for 
example, are mild and some severe, and a person’s resistance 
against an attack is partly dependent upon the vigor of the 
germs. 
Now, this resisting power is clearly located in the living cells of 
the body and is dependent upon their normal functions. It is 
only the living cell which can resist the invasion of microdrgan- 
isms, either wholly or partially. From this it follows that the 
resistance will be greatest when the body cells are in the highest 
state of physical activity, and will diminish when they become 
somewhat impaired in vitality. Anything which tends to reduce 
the physical health of the individual tends to reduce his power 
of resistance. For example: sometimes an individual shows a 
great tendency to develop boils or abscesses, and but little power 
of resisting them. We say his ‘“‘blood is in a bad condition.” 
By this is really meant that his body activities are so repressed 
that he is unable to resist the invasion of some of the common 
bacteria which are present on every hand and which an individual 
in healthy condition easily repels. If his physical vigor can be 
restored, the troubles will disappear, although the bacteria which 
produce the boils and abscesses are just as abundant around him 
as before. Physical vigor is the best protection against the in- 
vasion of parasitic bacteria, and a weakened physical condition 
invites attack. 
This matter is emphasized here because it is too generally lost 
sight of in the combat against infectious diseases. During the 
first years of the study of bacteriology there was a very general 
tendency, in the attempt to avoid diseases, to place the whole 
emphasis upon the methods of avoiding bacteria. If a disease 
is caused by a bacterium, what more natural method could be 
suggested for avoiding it than to avoid the bacterium? In 
accordance with this idea there developed a long series of rules 
