40 Manual of Veterinary Microbiology. 



upon animals and mankind ; they are called pathogenic 

 when, by their pullulation, they give rise to diseases. 

 These afi'ections have long been known but the study 

 of their causes, the infinitely little, is of quite recent 

 date ; such diseases are called infectious. Later, we 

 will have to distinguish infections properly so called 

 from intoxications of microbic origin. 



The term virulence is applied to the collection of 

 properties by which pathogenic microbes are able to 

 prove detrimental to living beings. This faculty is 

 far from being unchangeable ; it may present all de- 

 grees of intensity, in accordance with conditions ca- 

 pable of modifying the vitality of the germ. 



The term virus is applied to solid, liquid, or gaseous 

 vehicles containing pathogenic germs. 



I. Conditions of life of pathogenic germs in external 

 media and within the living organism. 



Those germs which cause diseases do not exclusively 

 and necessarily live within the economy. Some of 

 them can also multiply in dead organic media ; others, 

 indeed, only pullulate within the organism incidentally 

 the external media being really their natural field of 

 multiplication. 



In accordance with these considerations pathogenic 

 germs have been divided into three categories : 



a. Contagious obligatory parasites ; 



b. Contagious facultative parasites ; 



c. Non-contagious facultative parasites. 



Contagious obligatory parasites. — These are repre- 

 sented by microbes which, under natural conditions, 

 only reproduce themselves in the living organism and 

 are transmitted from one animal to another without 



