ing of disease is left largely to the physician,
much is to be gained through a more general knowledge of its causes and
the methods of its prevention.

*Causes of Disease.*--Disease, which is some _derangement of the vital
functions_, may be due to a variety of causes. Some of these causes, such
as hereditary defects, are remote and beyond the control of the
individual. Others are the result of negligence in the observance of
well-recognized hygienic laws. Others still are of the nature of
influences, such as climate, the house in which one lives, or one's method
of gaining a livelihood, that produce changes in the body, imperceptible
at the time, but, in the long run, laying the foundations of disease. And
last, and most potent, are the minute living organisms, called microbes or
germs, that find their way into the body. Although there are two general
kinds of germs, known as _bacteria_ (one-celled plants) and _protozoa_
(one-celled animals), most of our germ diseases are caused by bacteria.

*Effects of Germs.*--While there are many kinds of germs that have no ill
effect upon the body and others that are thought to aid it in its work,
there are many well-known varieties that produce effects decidedly
harmful. They gain an entrance through the lungs, food canal, or skin,
and, living upon the fluids and tissues, multiply with great rapidity
until they permeate the entire body. Not only do they destroy the
protoplasm, but they form waste products, called _toxins_, which act as
poisons. Diseases caused by germs are known as infectious, or contagious,
diseases.(129) The list is a long one and includes smallpox, measles,
diphtheria, scarlet fever, typhoid fever, tuberculosis, la grippe,
malaria, yellow fever, and others of common occurrence. In addition to the
diseases that are well pronounced, it is probable that germs are
responsible also for certain bodily ailments of a milder character.(130)

*Avoidance of Germ Diseases.*--The problem of preventing diseases caused by
germs is an exceedingly difficult one and no solution for all diseases has
yet been found. One's chances of avoiding such diseases, however, may be
greatly enhanced:

1. By strengthening the body through hygienic living so that it offers
greater resistance to the invasions of germs.

2. By living as far as possible under conditions that are unfavorable to
germ life.

3. By understanding the agencies through which disease germs are spread
from person