CHAPTER III. 

 INFECTION. 



INFECTION is the successful invasion of the organism by 

 parasites. Unfortunately custom has partly sanctioned the 

 use of the word infection in various other and sometimes 

 confusing senses. Thus, a table or knife upon which bacteria 

 are known to be or are even supposed to be present, is said by 

 the surgeon to be "infected," when infective might better 

 convey the meaning intended. The mouth and intestine, 

 which naturally harbor numerous bacteria of various forms, 

 are also spoken of by the surgeon as "infected," when no 

 abnormal condition is present. A splinter penetrating the 

 skin, and carrying harmless bacteria into the deeper tissues, 

 is said to infect them, when, in fact, none of the phenomena 

 of infection result. 



Infection should always imply an abnormal state resulting 

 from the deleterious action of the parasite upon the host. 

 The colon bacillus is a constant inhabitant of the intestine of 

 every human being, and of most of the lower animals, but 

 under ordinary conditions does no harm. The intestine is, 

 therefore, not "infected" but infested with it, and it is only 

 when abnormal or unnatural conditions arise that infection 

 can take place. The constant association of certain bacteria 

 with certain parts of the body to which they ordinarily do no 

 harm, but into which they may rapidly invade when appro- 

 priate conditions arise, may best be described, as suggested 

 by Adami, as sub-infection. The potentiality is always 

 there, though active invasion is but rarely if ever accom- 

 plished. 



There are two inseparable factors to be considered in all 

 infections : the organism infecting, and the organism infected. 

 The first is the parasite, the second, the host. Infectivity 

 and infectability may depend upon peculiarities of either 

 parasite or host. Organisms that have lived together in a 

 neutral relationship for an almost indefinite period may 

 suddenly cease their customary association, because of 



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