4 INFECTION 



parts of the body, where they may become localized, multiply, 

 and give rise to any one of many forms of lesions. It may 

 happen that the point of entrance is so obscure that the result- 

 ing morbid changes are not easily traced to an external 

 infection. There are many illustrations of this in comparative 

 pathology, such for example as suppurative cellulitis. For 

 convenience in discussion, infections may be divided into two 

 clinical groups, namely : wound infections and specific infec- 

 tious diseases, although in certain instances they cannot be 

 separated. 



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

 tions, it is well not to be too closely circumscribed by classifi- 

 cations. 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 fol- 

 lowing an infection are simply the results of parasitism. 



In the study 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 

 conditions, become parasitic and cause infections resulting in 

 more or less local or general disturbance. Many 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 multiply 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 many species of bacteria ordinaril}^ 

 considered harmless with which the animal body is constantly 

 surrounded. In the search for the cause of many lesions sup- 

 posed from their nature to be infectious, or in applying meth- 

 ods for their prevention, it is well to take into consideration all 

 microorganisms which might possibly be the causative factors 

 and not limit the search to the detection of the already recog- 

 nized pathogenic species. Recent investigations point to the 

 conclusion that domesticated animals frequently suffer as the 



