Sources of Infection 79 



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 para- 

 sitic when they live in or upon them. Saprophytic bacteria 

 when accidentally transplanted from their natural environ- 

 ment 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 mul- 

 tiply rapidly, invade the tissues in all directions, eliminate 

 their metabolic products into the juices, and occasion vary- 

 ing morbid conditions. 



The parasitic bacteria live in habitual association with 

 higher organisms. Sometimes, and indeed most commonly, 

 it is a harmless association, like that of certain cocci upon 

 the skin, but occasionally it results in the destruction of 

 the tissues and the death of the host, as in tuberculosis, 

 leprosy, etc. 



The group of pathogenic organisms has no well-defined 

 limits, for it is frequently observed that micro-organisms well 

 known under other conditions, and not known to have been 

 engaged in pathogenic processes, turn up unexpectedly as 

 the cause of some morbid condition. Indeed, although we 

 are acquainted with -u large number of organisms that have 

 never been observed in connection with disease, we are scarcely 

 justified in concluding that they are incapable of producing 

 injury should proper conditions arise. 



SOURCES OF INFECTION, 



The sources of infection may be exogenous or endogenous; 

 that is, they may arise through the admission to the tissues 

 of micro-organisms from sources entirely apart from the 

 individual infected, or through the admission of some of 

 those parasitic and usually harmless organisms constantly 

 associated with him. 



Exogenous infections arise through accidental contact 

 with infective objects belonging to the external world. 



A polluted atmosphere may carry into the respiratory 

 passages micro-organisms capable of colonizing there. 

 Polluted water or food may carry into the intestine micro- 

 organisms whose temporary residence may entirely change 

 the functional and structural integrity of the parts. 



