CLASSIFICATION.--We may divide diseases into three classes: _non-specific,
specific_ and _parasitic_.

_Non-specific diseases_ have no constant cause. A variety of causes may
produce the same disease. For example, acute indigestion may be caused by a
change of diet, watering the animal after feeding grain, by exhaustion and
intestinal worms. Usually, but one of the animals in the stable or herd is
affected. If several are affected, it is because all have been subject to
the same condition, and not because the disease has spread from one animal
to another.

_Specific Diseases._--The terms infectious and contagious are used in
speaking of specific diseases. Much confusion exists in the popular use of
these terms. A _contagious_ disease is one that may be transmitted by
personal contact, as, for example, influenza, glanders and hog-cholera. As
these diseases may be produced by indirect contact with the diseased animal
as well as by direct, they are also _infectious_. There are a few germ
diseases that are not spread by the healthy animals coming in direct
contact with the diseased animal, as, for example, black leg and southern
cattle fever. These are purely infectious diseases. Infection is a more
comprehensive term than contagion, as it may be used in alluding to all
germ diseases, while the use of the term contagion is rightly limited to
such diseases as are produced principally through individual contact.

_Parasitic diseases_ are very common among domestic animals. This class of
disease is caused by insects and worms, as for example, lice, mites, ticks,
flies, and round and flat worms that live at the expense of their hosts.
They may invade any of the organs of the body, but most commonly inhabit
the digestive tract and skin. Some of the parasitic insects, mosquitoes,