476 FARM ANIMALS 
memorial it has been known that certain diseases were 
transferable from the diseased to the healthy. In many 
cases the older observers believed in the theory that there 
was “something in the air,” whereby the maladies were 
communicated, and people or animals got sick. The 
science of bacteriology has since explained the reason. 
We know now that every disease of an infectious nature 
is caused by a specific micro-organism which in one way 
or other, must be introduced into the animal body to 
cause that disease. The principal ways by which such 
infection takes place are: (1) Through the digestive tract, 
in the food or drink. (2) Through the respiratory tract, 
inhaling air containing the discase-producing organisms. 
(3) Through abrasions of the skin, where organisms 
may be admitted in pricks, scratches, cuts or sores. (4) 
Through the bites of insects. All infective diseases are 
due to the presence of the invading organisms either cir- 
culating in the blood or manufacturing toxic material 
at the seat of injury, the poison then circulating in the 
blood. It is obvious that control of such diseases lies 
more in preventive measures than in treatment and cure 
afterwards. This is to be done by keeping unexposed 
animals from the vicinity of infected places; by keeping 
stock in good health and vigor; by providing pure food 
and drink, and exercise; and by housing in sanitary quar- 
ters where much sunlight and fresh air at all times are 
admitted. Furthermore, it is necessary to make certain 
that infectious germs are not carried to the flock or herd, 
either by affected stock, or by means of clothing, drink- 
ing vessels, feeding troughs, food, water, dogs, birds, 
bedding or other media by means of which these germs 
may be transmitted from affected individuals to healthy 
herds or flocks. 
