INFECTION AND IMMUNITY. 
33 
breathed. When these bacteria gain access to the 
body, only those survive that find the conditions favor¬ 
able for their existence. For this reason it is found 
that each cavity or portion of the body harbors a 
group of bacteria peculiar to it. The varieties of bac¬ 
teria found in the saliva, for example, are quite dif¬ 
ferent from those found in the intestine. Most of 
these constant bacteria of the body are harmless, but 
some pathogenic forms occur which manifest their 
power to produce disease only when some injury 
affords a point of entrance to the tissues or the re¬ 
sistance of the individual is lowered. Thus in the 
skin there may be many kinds of bacteria, the most 
important of which are the pus-forming cocci, the 
staphylococci, and streptococci. They do no harm 
under normal conditions, but if there is any injury 
to the skin these organisms may enter and give rise to 
a boil, an abscess, or erysipelas. It is mainly against 
these pus-forming bacteria that the preparation of the 
patient before operation is directed. Unfortunately 
these bacteria live actually in the skin, that is, below 
the surface; so that skin disinfection must be very 
thorough to be effectual and, even under most favor¬ 
able conditions, cannot be considered as absolute. 
In the air-passages large numbers of bacteria are 
found which enter with the air breathed in. Most of 
them are caught on the moist surfaces of the mouth, 
throat, and nose; very few if any ever reach the lungs 
directly through the trachea and bronchi. In the 
mouth the pneumococci, staphylococci, and strepto- 
3 
