INFECTION. 129 
such as tetanus, anthrax, etc., being able to withstand 
destruction for a long time, retain their power of pro- 
ducing infection for months or even years after elimi- 
nation from the body. The bacteria which form no 
spores show great variation in their resistance to out- 
side influences. Some of these, such as the influenza 
bacilli and the gonococci, the virus of syphilis and 
hydrophobia, are extremely sensitive; the pneumococci, 
cholera spirilla, glanders bacilli, etc., area little hardier; 
then follow the diphtheria bacilli, and after them the 
typhoid and tubercle bacilli and the staphylococci. 
4. The ability or the lack of ability to grow outside 
of the infected tissues. 
Such bacteria as the pneumococcus, tubercle, influ- 
enza, and lebrosy bacilli do not develop, as far as we 
know, outside of the body under ordinary conditions. 
Others, like the diphtheria bacillus and the strepto- 
coccus, may under certain conditions, as in milk in 
warm places, develop and produce infection. Others, 
again, such as the streptococcus and staphylococcus, 
typhoid and anthrax bacillus, the cholera spirillum, 
and some anaérobics, may develop under peculiar con- 
ditions existing in water or soil. 
While for the pathogenic bacteria, as a rule, the 
saprophytes met with in the soil and water are antago- 
nistic, yet in some cases—and especially is this true of 
the anaérobic bacteria—they are helpful. Such bacilli 
as tetanus are believed to require the associaton of an- 
aérobic bacteria to permit of their development in the 
soil in the presence of oxygen. 
A large number of the infectious bacteria are able to 
develop in or upon some portion of the skin or mucous 
membrane, either after or before disease, and without 
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