CHAPTER III. 



PHENOMENA OF INFECTION. 



There are two view-points from which the production 

 of an infectious disease must always be studied, viz., 

 (i) the infecting micro-organism, and (2) the state of 

 the body at the time the infection occurs. Apart from 

 injuries and direct poisonings, there are no independent 

 causes of disease. When we say an infectious disease 

 is due to this or that micro-organism, we are making 

 a statement which expresses only a fraction of the true 

 sequence of events, since unassisted, infectious agents 

 probably never, under the ordinary conditions of life, 

 bring about disease. To be sure, their presence in an 

 infectious process is indispensable, yet the mere fact 

 of their presence counts for naught unless the body 

 has been prepared, so to speak, for their invasion. 

 Furthermore, even in the event that the body be in a 

 most favorable condition for infection, there are several 

 conditions which micro-organisms must fulfill before 

 they can exercise their specific functions. For the 

 foregoing reasons, therefore, an infectious disease is the 

 result of the co-existence of a chain of fortuitous phenom- 



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