194 PRINCIPLES OF VETERINARY SCIENCE 



a nim al body. Simply finding these bacteria on the surface or 

 in the tissues of the body does not constitute infection. 



In order to produce infection, the pathogenic organisms them- 

 selves, or their toxic products, must penetrate into the tissues of 

 the body and feed on its proteins. Under natural conditions 

 few pathogenic organisms grow and multiply outside the body, 

 although they may exist for some time. Any object on which 

 they may be brought into the body is said to be infective. The 

 body into which they are brought is said to be infected. The 

 organisms through which the infection is brought are said to be 

 infectious. This term is also applied to that class of diseases 

 caused by these organisms. 



The pathogenic properties of an infectious organism may be 

 summed up in the word virulence. By this is meant its ability 

 to produce disease. There is as much variation in the capability 

 of growth and toxin production of a given organism as there is in 

 those characteristic in different animals. The virulence may be 

 intensified or weakened depending upon the resistance offered 

 by the infected animal. For example, foot-and-mouth disease 

 in some epizootics runs an unusually severe course and has a high 

 mortality, while in most outbreaks it is not a fatal disease, 

 recovery taking place in the course of two weeks. 



Although it is desirable to know the exact meaning of these 

 terms, the point of practical importance is to understand how 

 diseases are transmitted rather than to quibble about the terms 

 to be used. 



The entrance of infectious organisms in the body is usually by 

 one of the following paths: The broken skin (cut or wound); 

 the digestive tract (food or drink) ; the respiratory tract (breath- 

 ing in germs); the genital tract (sexual intercourse). In the 

 new-born animal the raw navel forms an important place for 

 both the entrance and the propagation of bacterial organisms. 

 Most infectious agents are incapable of producing infection 

 unless they gain entrance by a particular path. For example, 

 the tetanus bacillus is harmless when taken into the stomach 

 even in large numbers, while a very few will produce tetanus 

 when introduced under the skin. 



The dissemination of infectious organisms in the body varies 

 with the kind, number, and virulence of the bacteria. Some 

 bacteria remain at or near the site of infection (tetanus) ; others 



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