EXOGENOUS AND ENDOGENOUS INFECTIONS 85 



There are no hard-and-fast rules that can be set down in classifying 

 bacterial infections; bacteria that are commonly transmitted by one 

 means may, under slightly altered conditions, be transmitted by another. 

 The usual classification, by which certain diseases are classified as con- 

 tagious and others as infectious, should be abolished, and all should be 

 grouped under the term infectious, there being a definite understanding of 

 those cultural characteristics that render infection more likely to occur 

 by direct and immediate contact, and those that may occur in an in- 

 direct or roundabout manner. It may therefore be stated that all bac- 

 terial diseases are infectious; the term contagious may be reserved for 

 those spread or contracted as the result of direct contact. 



EXOGENOUS AND ENDOGENOUS INFECTIONS 



Infection may occur as the result of the admission of microparasites 

 to the tissues from sources entirely apart from the individual infected 

 (exogenous infection), or from the admission of some of those micro- 

 parasites living normally and harmlessly on the skin and adjacent mucous 

 membranes, and which, under special conditions, have assumed patho- 

 genic properties (endogenous infections). 



Exogenous infections are the more usual form, and result from con- 

 tact with infective material outside the body. 



1. Microorganisms, such as typhoid and cholera bacilli, which can 

 live for varying periods of time in water and foods, are particularly 

 likely to gain entrance through the gastro-intestinal tract. Micro- 

 organisms may be present in milk derived directly from diseased animals 

 or tissues, and when ingested, may produce disease. Thus, for example, 

 the germs of tuberculosis may be conveyed in either milk or flesh, young 

 children being particularly exposed to this method of infection. 



2. The atmosphere may be laden with microorganisms, which, 

 whether or not capable of proliferating outside of the body, are prone 

 to gain entrance through the respiratory tract, especially through the 

 upper air-passages, the pharynx and tonsils being often the seat of the 

 infection. 



3. Microorganisms capable of existing on the skin may gain entrance 

 to the deeper tissues as the result of wounds. Under these conditions 

 of lowered vitality of the local tissues microorganisms that would 

 otherwise be harmless may become pathogenic and morbidly affect the 

 host, either locally or generally. As the skin is brought so freely in 

 contact with external objects, various microorganisms, and particularly 



