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 o-r 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- 



