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BIOLOGY OF SPECIALIZED GROUPS 



which are produced by a definite bacterium or microorganism. 

 The symptomatology of these diseases is equally specific. 

 The definite bacterium or microorganism is the only exciting 

 cause of this class of diseases. Diphtheria, typhoid fever, 

 bubonic plague, are types of specific infectious diseases. The 



FIG. 50. Treponema pallidum of syphilis in tissue. Williams. 



etiological agents in these diseases are Bact. diphtherias, 

 B. typhosus, and B. pestis, and natural infection of a body with 

 these bacteria usually produces a definite pathological con- 

 dition and symptomatology. Some infectious diseases, in 

 which the etiological agent is not known, are called specific 

 infectious diseases. Hydrophobia, smallpox, and chicken- 

 pox are examples of this class. 



