128 DISEASES OP SHEEP. 



only single individuals from special causes, as, for example, 

 inflammation. Sporadic diseases may, however, sometimes 

 assume the character of a plague, and vice versa. 



With regard to the mode of spreading into contagious and 

 non-contagions disease. 



Contagious diseases are those in which matter is formed 

 and discharged, which may produce the same disease in 

 other healthy animals if it is transferred to them. This 

 matter is called " the virus of the disease." 



If the virus is contained in certain bodies, such as saliva, 

 blood, mucus, excrements, etc., it is called fixed (as in mor- 

 tification of the spleen and madness), and acts either by 

 being directly transferred to the body of a healthy animal 

 (by licking, biting, etc), or by the healthy animals coming 

 in contact with objects contaminated with it (such as har- 

 ness, blankets, cribs, mangers, etc.). 



If the virus separates as a vapor, as by the exhalations 

 of the skin and the respiration, it is thus mixed with the 

 air and breathed by other animals. Winds sometimes carry 

 it to a considerable distance. It acts probably by being 

 inhaled and by coming thus in contact with the blood. The 

 fixed virus is mostly produced by chronic diseases not at- 

 tended by fever, and the volatile by feverish diseases. In 

 many, a fixed and a volatile virus are developed at the 

 same time. Many kinds of virus act only on the species in 

 which they are developed (as in the rinderpest and pleuro- 

 pneumonia), while others may be transferred to other animals 

 (such as madness and mortification of the spleen). 



Some contagious diseases attack an animal but once, as 

 the sheep pox, rinderpest, etc., and the animal is then no 

 longer susceptible for their virus. 



Non-contagious diseases are those in which no virus is 

 formed. They comprise by far the most numerous class ; 



