SPECIFIC CAUSE.--The specific cause of hog-cholera is an _ultra-visible
organism_ that is present in the excretions, secretions and tissues of a
cholera hog. De Schweinitz and Dorset in 1903 produced typical hog-cholera
by inoculating hogs with cholera-blood filtrates that were free from any
organism that could be demonstrated by microscopical examination or any
cultural method. The term ultra-visible virus is applied to the virus of
hog-cholera.

_The ultra-visible virus_ is eliminated from the body of the cholera hog
with the body secretions and excretions. Healthy hogs contract the disease
by eating feed or drinking water that is infected with the virus. There are
other methods of infection, but field and experimental data show that
hog-cholera is commonly produced by taking the germs into the body with
food and drinking water.

ACCESSORY CAUSES.--The usual method of introducing hog-cholera into a
neighborhood is through the importation of feeding or breeding hogs that
were infected with the disease before they were purchased, or became
infected through exposure to the disease in the public stock-yards and
stock-cars. The shipping of feeding hogs from one section of the country to
another, and from public stock-yards, has always been productive of
hog-cholera. Dr. Dorset states that more than fifty-seven per cent of the
hog-cholera outbreaks are caused by visiting, exchanging work, exposure on
adjoining farms and harboring the infection from year to year (Fig. 79),
and more than twenty-three per cent to purchasing hogs and shipping in
infected cars, birds and contaminated streams.

[Illustration: FIG. 79.--A hog yard where the disease-producing germs may
be carried over from year to year.]