60 Infection 



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. 



Pathogenic bacteria are associated with the production 

 of morbid conditions. They may be saprophytes acciden- 

 tally admitted to the body, or they may be parasites un- 

 known except in the body of the host. The group of patho- 

 genic 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 patho- 

 genic processes, turn up unexpectedly as the cause of some 

 morbid condition. Indeed, although we are acquainted 

 with a large number of organisms that have never been 

 observed in connection with disease, we are scarcely justi- 

 fied 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 



