CHAPTER III. 

 INFECTION. 



INFECTION is the successful invasion of the 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 harmless bacteria 

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

 "injected," when, in fact, it would be correct to describe 

 these objects as infective. 



Infection, therefore, implies 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 " infected," but infested with it, and it is 

 only when abnormal or unnatural 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 appro- 

 priate 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 cus- 

 tomary association, because of newly acquired power of in- 

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



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