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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