, produce acids and other materials
injurious to themselves. Furthermore, lack of proper food, moisture, or
favorable temperature, and competition with other organisms tend to
prevent their unrestricted growth and multiplication.

[Illustration: FIG. 5.--BACTERIA WITH SPORES. (_Williams._)]

SPORES.--Most bacteria die if conditions become unfavorable to their
growth, but some enter into a resting stage. This stage is characterized
by the development of round or oval glistening bodies called spores,
which are of dense structure and possess an extraordinary power to
withstand heat, chemicals, and unfavorable surroundings. Except in rare
instances a single cell produces but one spore. As soon as favorable
conditions of temperature, moisture, and food supply are restored, the
spore develops into the active form of the germ; it may, however, remain
dormant for months or years. Spore formation, however, occurs in only a
very few varieties of pathogenic bacteria.

DISTRIBUTION.--Bacteria are very widely distributed in nature; they are
in fact found practically everywhere on the surface of the earth. They
are present in plants and water and food; on fabrics and furniture,
walls and floors; and they are found in great numbers on the skin, hair,
many mucous surfaces, and other tissues of the body.

PROTOZOA

The protozoa are the lowest group of the animal kingdom. Like bacteria
they are composed of single cells so small as to be visible only under
the microscope. They play an important part in causing certain diseases
of man, especially in the tropics. Among the well-known human diseases
of protozoan origin are malaria, amoebic dysentery, and
sleeping-sickness. Protozoa also cause several wide-spread and serious
plagues of domestic animals.

VISIBLE PARASITES

A few diseases are caused by parasites large enough to be seen with the
naked eye. One of the most important is hookworm disease. This disease
is caused by a tiny worm which penetrates the victim's skin and
ultimately finds its way into the intestine. Other diseases also are
caused by parasitic worms, such as tapeworms, pinworms, and trichinae.
The latter are acquired as a result of eating infected meat,
particularly infected pork that has not been thoroughly cooked.

TRANSMISSION OF PATHOGENIC ORGANISMS

Pathogenic or disease producing organisms need for their development
food, moisture, darkness, and warmth, conditions that exist within the
human body. Wh