PART IV. 



GENERAL PATHOGENIC BACTERI- 

 OLOGY. 



CHAPTER XXII. 

 INTRODUCTION. 



Pathogenic bacteriology treats of the unicellular micro- 

 organisms which are responsible for disease conditions, i. e., 

 pathological changes in other organisms. Hence not only 

 are bacteria considered, but also other low vegetable forms, 

 as yeasts and molds, likewise protozoa in so far as they 

 may be pathogenic. For this reason the term pathogenic 

 "Microbiology" has been introduced to include all these 

 organisms. It is largely for the reason that the methods 

 devised for the study of bacteria have been applied to 

 the investigation of other microorganisms that the term 

 "bacteriology" was extended to cover the entire field. The 

 general discussion in this chapter is intended to include, 

 therefore, microorganisms of whatever kind pathogenic to 

 animals. 



The term pathogenic as applied to an organism must be 

 understood in a purely relative sense, since there is no single 

 organism that can cause disease in all of a certain class, but 

 each is limited to a more or less narrow range. Some form 

 of tuberculosis attacks nearly all vertebrates, but no other 

 classes of animals and no plants. Lockjaw or tetanus 

 attacks most mammals, but not any other vertebrates 

 naturally. Typhoid fever affects human beings; hog cholera, 



( 227 ) 



