flies and ticks, act as secondary hosts for certain animal microorganisms
that they transmit to healthy individuals through the punctures or the
bites that they are capable of producing in the skin.

CAUSES.--For convenience we may divide the causes of disease into the
predisposing or indirect, and the exciting or direct.

_The predisposing causes_ are such factors as tend to render the body more
susceptible to disease or favor the presence of the exciting cause. For
example, an animal that is narrow chested and lacking in the development of
the vital organs lodged in the thoracic cavity, when exposed to the same
condition as the other members of the herd, may contract disease while the
animals having better conformation do not (Fig. 1). Hogs confined in
well-drained yards and pastures that are free from filth, and fed in pens
and on feeding floors that are clean, do not become hosts for large numbers
of parasites. Hogs confined in filthy pens are frequently so badly infested
with lice and intestinal worms that their health and thriftiness are
seriously interfered with. In the first case mentioned the predisposition
to disease is in the individual, and in the second case it is in the
surroundings (Fig. 2).

[Illustration: FIG. 1.--Side and posterior view of bull showing
conformation favorable to the development of disease.]

_The exciting causes_ are the immediate causes of the particular disease.
Exciting causes usually operate through the environment. With the exception
of the special disease-producing germs, the most common exciting causes are
faulty food and faulty methods of feeding. The following predisposing
causes of disease may be mentioned: