SECTION IV 



PATHOGENIC MICROORGANISMS, EXCLUSIVE OF THE 



PROTOZOA 



CHAPTER XX 



MICROORGANISMS AS A CAUSE OF DISEASE 



Infectious Diseases. An infectious disease is one caused by a 

 microorganism. In general, the presence of microorganisms in the 

 body does not constitute infection unless proliferation and mul- 

 tiplication occurs and there are pathological changes in tissues. 

 An individual harboring such an organism is called the host, 

 and is said to be infected. The latter expression is commonly 

 used also to describe any object, such as a surgical instrument, 

 which may introduce pathogenic organisms into the body; a better 

 term for such, however, is infective. An infective object, such as 

 the clothing of a diphtheritic patient or the manger of a glandered 

 horse, is sometimes called a fomite. An organism which will 

 produce disease is sometimes spoken of as a virus; it is customary 

 to speak of the cause of a disease where the infecting agent is 

 not known by this name. By no means all diseases are infectious. 

 Among the non-infectious diseases may be named azoturia, dia- 

 betes, Bright's disease, etc. 



Contagious Diseases. Any disease in which the causal organ- 

 ism may be readily transferred from one individual to another by 

 direct or indirect contact is said to be contagious. The terms in- 

 fectious and contagious have been used very loosely, particularly 

 in veterinary writings, sometimes even interchangeably. The 

 best usage, however, is strictly to limit the terms as here defined. 

 Infectious has to do with the cause of a disease, and contagious 

 with its ease and method of transmission. All infectious diseases, 



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