from the body. If unclean hands could be kept away from the
orifices of the body, particularly the mouth, many diseases would soon
cease to exist.

Defenses of the Body

In view of all the dangers from disease-producing germs it may seem
surprising that the human race has not long ago succumbed to its
invisible enemies. But the body has various defenses by means of which
it may prevent invasion, or successfully combat its enemies in case they
do gain access.

The unbroken skin is usually impassable to bacteria. Virulent organisms
are often found upon the skin of perfectly healthy persons, where they
appear to be harmless unless an abrasion occurs which affords entrance
into the deeper tissues. Most bacteria breathed in with the air cling to
the moist surfaces of the air-passages and never reach the lungs.

Mucous membranes lining the mouth and other cavities of the body would
prove favorable sites for the growth of bacteria if the mucus secreted
by them were not frequently removed. The mouth of a healthy person may
contain bacteria of many kinds, but the saliva has a slight disinfectant
power and serves as a constant wash to the membranes. The normal gastric
(stomach) juice is decidedly unfavorable to the growth of bacteria,
although it does not always kill them; they often pass through the
stomach and are found in large numbers in the intestines. Other bodily
secretions, such as the tears and perspiration, tend to discourage
bacterial growth.

Tissues of the body vary greatly in their power to resist invading
germs, so that the route by which germs enter influences the severity of
their effects. Typhoid bacilli and the spirilla of Asiatic cholera when
taken with food or water produce far more serious disturbances than when
injected under the skin; infections from pus germs through an abrasion
of the skin may result in a slight local disturbance, while the same
amount introduced into a deeper wound might cause a fatal infection.
Certain germs nourish in certain tissues only; even tuberculosis, which
attacks practically all tissues, has its favorite locations.

IMMUNITY.--In addition to its mechanical defenses against disease, the
body shows a varying degree of _immunity_, or the power possessed by
living organisms to resist infections. Immunity or resistance is the
opposite of susceptibility. It is exceedingly variable, being greater or
less in different people and under different conditions, but the exact
way