Many types of bacteria can cause infection, and the
capable of carrying on all functions of life, including
powerful toxins produced by some can be used as
growth and reproduction. Microorganisms do not have
a digestive tract, organs of sight, or a heat-regulating
caused by bacteria are typhoid fever, meningitis, and
system. Many of them resemble plant life and are
regarded as being in the vegetable kingdom. Some
m i c r o o rga n i s m s , s u c h a s t h e p r o t o z o a , h ave
2. Rickettsiae are usually smaller than bacteria, but
characteristics that place them in the animal kingdom.
they are still visible through an ordinary microscope.
They grow only within living cells, and they are potent
Microorganisms are universally distributed in the
disease producers in man and animals. Many of them
air, water, and soil. Every cubic foot of topsoil provides
are transmitted by insect bites. Examples of diseases
a natural home for billions of soil organisms. The skin,
caused by rickettsiae are Rocky Mountain spotted fever
hair, nose, mouth, and digestive tract of humans and
other animals harbor a considerable variety of
3. Viruses are even smaller than rickettsiae and are
microbes in large numbers.
not visible with the ordinary microscope. Some have
Microorganisms capable of producing disease are
been photographed through the electron microscope.
known as pathogens. Most of these pathogens are
Like the rickettsiae, they will grow only within the
parasites and live on or within another living organism,
living cell. Viruses and rickettsiae are probably less well
called a host, which provides shelter and nourishment.
distributed than bacteria because they are more
Other microorganisms thrive on decaying or dead
particular in their growth environments. However, it is
organic material and are known as saprophytes. Most
known that both can survive for short periods of time in
the air. Examples of virus diseases are mumps,
microorganisms are nonpathogenic; that is, they do not
smallpox, psittacosis (parrot fever), and influenza.
cause disease. In fact, many of them are beneficial to
both man and plant life. Certain microorganisms are
4. Fungi include such plants as yeasts, molds, and
responsible for producing many antibiotics, such as
mildews. These organisms are known for their ability to
penicillin and streptomycin. Others are important in
spoil foods and fabrics.
the production of alcoholic beverages, manufacture of
Generally speaking, diseases caused by fungi in
vinegar, leather making, and curing cheese and
humans are less severe than those produced by other
tobacco, as well as in the preparation of industrial
microorganisms. They usually produce low-grade,
mild, and often chronic diseases. A few fungi are
O n t h e b a s i s o f s t r u c t u r a l a n d b e h av i o r a l
capable of producing serious diseases, such as
characteristics, microorganisms are divided into five
blastomycosis (a chronic infection affecting the skin or
distinct classifications of biological warfare agents as
the lungs, bones, liver, spleen, and kidneys). Several
diseases of plants are caused by fungi. Examples are
potato blight, cotton root rot, corn smut, and wheat
rust. If an attack is made on food crops, the agents used
might be in this class.
5. Protozoa are single-celled, animal-like forms
that occur in a variety of shapes and often have
complicated life cycles. Some protozoa cause diseases
in both man and animals. Problems of production and
transmission limit their application in biological
A brief discussion of the characteristics of each of
warfare, but it must not be assumed that these problems
these microorganisms and their potential threat to
could not be solved. Examples of protozoa infections of
man are amoebic dysentery and malaria.
1. Bacteria are very small single-cell organisms.
VECTORS OF DISEASE.-- Disease vectors are
However, they are large enough to be visible through an
animal carriers that transfer infective agents from one
ordinary microscope. Bacteria may be spherical,
host to another. They usually are arthropods (insects,
rod-shaped, or spiral in form. They are present
arachnids, and crustaceans) but may be other animals.
everywhere in nature, in air, soil, water, and animal and
Disease vectors are classified into two types as follows:
plant bodies, both living and dead.