PART II 



CHAPTER IX 



Infection. In the early days of bacteriology the presence of 

 bacteria in or on the skin or mucous membranes was regarded as 

 an evidence of a diseased condition. It is well known now, how- 

 ever, that organisms such as streptococci, staphylococci, and 

 pneumococci are frequently present in the nose or mouth or on 

 the skin of normal healthy persons. The intestines contain many 

 thousands of different species, but only under unusual conditions 

 do they produce disease. The mere contact, therefore, of micro- 

 organisms with bodies of animals or man does not necessarily 

 mean a diseased condition. When, however, they pass the pro- 

 tective skin and membranes, invade the deeper tissues, and multiply 

 there they may produce poisons which give rise to the various 

 symptoms met with in disease. This invasion, multiplication, and 

 resulting disease is spoken of as an infection. 



In the production of an infection the main factors are (1) the 

 defensive forces the body can command to resist the invaders 

 and (2) the power of the invading organism to withstand all the 

 opposing forces the body can produce against it, to multiply and 

 to elaborate poisonous substances. 



Certain bacteria may live and multiply in the body apparently 

 without either causing or receiving injury. Such organisms may 

 be harmless and incapable of producing poisons or there may be 

 established between them and the body cells an equilibrium in 

 that the amount of poison produced is neutralized and rendered 

 inert by a corresponding amount of cell secretions. In the latter 



94 



