CHAPTER XVI 



CONTAGIOUS AND INFECTIOUS DISEASES 



CONTAGIOUS and infectious diseases are those that 

 are capable of transmission from one animal to another 

 of the same species. In some cases they may even be 

 transmitted to animals of another species, either by di- 

 rect contact, or through some medium which has been 

 infected by a sick animal. A contagious disease, as 

 commonly understood, is one that is "catching"; that is, 

 a well animal may contract it without coming into 

 actual contact with the diseased animal. It "goes in 

 the air." An infectious disease is contracted only when 

 the animal comes into contact with the diseased subject 

 or with some of its excretions. Amongst diseases of 

 the human subject, scarlet fever may be cited as an 

 example of a contagious disease, and typhoid fever of 

 an infectious one. Strictly speaking, however, it is 

 difficult to draw a distinct line between them, and there 

 is .now a tendency to use the term "transmissible" 

 rather than either of them. 



In many of the diseases of animals, the germ that 

 causes the particular disorder has been discovered, and 

 its nature is definitely known. Most of these germs are 

 microscopic living cells belonging to the lowest order of 

 plants. In a few diseases, the germ that causes the trouble 

 belongs to the lowest order in the animal kingdom. 



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