4 INFECTION 



In arriving at a clear understanding of the nature of in- 

 fections, it is well not to be too closelj- circumscribed by 

 classifications. It is better to look upon them as a series of 

 processes going on in the animal world due to the activities of 

 infecting or parasitic microorganisms. In other words, the 

 lesions following an infection or a specific infectious disease 

 are simpl}' the results of parasitism. 



In the stud}' of the various forms of infection in the lower 

 animals, lesions have been found to contain, apparently as their 

 causative factors, bacteria which suggest at least that certain of 

 the supposed saphrophytic organisms may, under certain con- 

 ditions, become parasitic and cause infections resulting in more 

 or less local or generalized disturbance. Manj' lesions seem to 

 be produced by bacteria which are harbored normally upon 

 the skin. When these organisms are introduced by accident 

 into the living tissues they multipl}- and acquire, if they did 

 not already possess it, the power to produce tissue changes. 

 We cannot, therefore, dismiss the subject of infection without 

 a passing consideration of the possible etiological significance, 

 under certain conditions, of man}- ordinarily considered harm- 

 less species of bacteria with which the animal body is con- 

 stantly surrounded. In the search for the cause of many le- 

 sions supposed from their nature to be infectious, or in apply- 

 ing methods for their prevention, it is well to take into consid- 

 eration all microorganisms whicli might possibly be the caus- 

 ative factors and not limit the search to the detection of the 

 already recognized pathogenic species. Recent investigations 

 point to the conclusion that frequently domesticated animals 

 suffer as the result of the invasion of bacteria at present not 

 listed among the pathogenic microorganisms, and what is true 

 in this regard for bacteria, may be hypothetically applied to 

 the higher fungi and to the protozoa. 



§ 3. Channels of Infection. There are a number of 

 ways by which microorganisms ma>- be introduced into the 

 living tissues of the animal body. The more common of these 

 areas follows, namely ; 



( I ) Through the digestive tract. Bacteria gain entrance 

 into the tissues from the digestive tract where they have been 



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