PART II 



CHAPTER VI 



INFECTION 



Infection is the successful invasion and growth of microorganisms in 

 the tissues of the body. 



The skin and adjacent mucous membranes contain numerous micro- 

 organisms, and under normal conditions these may invade the tissues, 

 but they are usually quickly destroyed and unable to proliferate, so that 

 mere invasion does not necessarily constitute infection. 



Unfortunately, custom has sanctioned the use of the term infection 

 as synonymous with contamination. The bacteriologist may speak of 

 the air, water, or his culture medium as being infected when they con- 

 tain microorganisms, or, in other words, are not sterile; similarly the 

 surgeon may speak of a knife or splinter of wood as being infected 

 whereas, while these may be infective or capable of producing infection, 

 it is more accurate to speak of them as being contaminated. In the 

 early days of bacteriology, the mere presence of microorganisms in or 

 on the skin and mucous membranes was regarded as equivalent to in- 

 fection. It is now well known that a person may harbor various micro- 

 organisms, such as staphylococci, streptococci, and pneumococci, with- 

 out apparent injury to the host, and this surface contamination, or even 

 occasional invasion of the tissues, does not necessarily indicate that the 

 host has been, is, or will be ill. 



Definition. When, however, microorganisms have passed the normal 

 barriers of the skin or mucous membranes and have invaded and prolifer- 

 ated in the deeper tissues, the process is spoken of as an infection. 



By common consent, the term infestation, or infestment, is being 

 applied in a similar manner to the presence and growth of animal 

 parasites; thus the intestine may be infected with Bacillus typhosus and 

 infested by Taenia saginata. 



A microorganism may be intimately associated with and have its 

 normal habitat in a certain part of the body and do no harm until special 

 conditions arise, when it may rapidly invade the tissues and produce 

 infection; this condition has been described by Adami as a subinfection, 

 and is illustrated by the constant presence of staphylococci and strepto- 

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