_Age_ is an important factor in the production of disease. Young and
immature animals are more prone to attacks of infectious diseases than are
old and mature animals. Hog-cholera usually affects the young hogs in the
herd first, while scours, suppurative joint disease and infectious sore
mouth are diseases that occur during the first few days or few weeks of the
animal's life. Lung and intestinal parasites are more commonly found in the
young, growing animals. Old animals are prone to fractures of bones and
degenerative changes of the body tissues. As a general rule, the young are
more subject to acute diseases and the old to chronic diseases.

[Illustration: FIG. 2.--Insanitary yards.]

_The surroundings or environments_ are important predisposing factors. A
dark, crowded, poorly ventilated stable lowers the animal's vitality, and
renders it more susceptible to the disease. A few rods difference in the
location of stables and yards may make a marked difference in the health of
the herd. A dry, protected site is always preferable to one in the open or
on low, poorly drained soil. The majority of domestic animals need but
little shelter, but they do need dry, comfortable quarters during wet, cold
weather.

_Faulty feed and faulty methods of feeding_ are very common causes of
diseases of the digestive tract and the nervous system. A change from dry
feed to a green, succulent ration is a common cause of acute indigestion in
both horses and cattle. The feeding of a heavy ration of grain to horses
that are accustomed to exercise, during enforced rest may cause liver and
kidney disorders. The feeding of spoiled, decomposed feeds may cause
serious nervous and intestinal disorders.

_One attack of a certain disease_ may influence the development of