CHAPTER III 

 INFECTION 



INFECTION is the successful invasion of an organism by micro- 

 parasites. Unfortunately custom has sanctioned the use of the 

 word in other and sometimes confusing senses, thus, a table or knife 

 upon which micro-organisms are known to be or are even supposed 

 to be; the mouth and intestine, which naturally harbor bacteria of 

 various forms, or a splinter penetrating the skin and carrying harm- 

 less bacteria into the deeper tissues, are all said by the surgeon to be 

 "infected" when, in fact, it would be more correct to describe them 

 as infective. 



The term infection should imply an abnormal state resulting from 

 the deleterious action of the parasite upon the host. The colon 

 bacillus is a harmless commensal of the intestine of every human 

 being, and of most of the lower animals. The intestine is not " in- 

 fected," but infested with it, and it is only when abnormal or un- 

 natural conditions arise that infection can take place. This form 

 of association of certain bacteria with certain parts of the body to 

 which they do no harm, but into which they may rapidly invade 

 when appropriate conditions arise, is described by Adami as sub- 

 infection. The possibility of infection is always there, though it 

 is but rarely that conditions arise under which it can be accomplished. 



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 as commensals, that is, in a state of neutral 

 relationship for an almost indefinite period, may suddenly cease 

 their customary association, because of newly acquired power of 

 invasion on the one hand, or diminished vital resistance on the 

 other, and infection take place where it had previously been 

 impossible. 



Bacteria are commonly called saprophytic when they live in nature 

 apart from other living organisms, and parasitic when they live in or 

 upon them. Saprophytic bacteria when accidentally transplanted 

 from their natural environment to the body of some animal, for 

 example, may or may not be capable of continuing life under the new 

 conditions. In the greater number of cases they die, but sometimes 

 the new environment seems better than the old, and they multi- 

 ply rapidly, invade the tissues in all directions, eliminate their met- 



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