The Causes of Infection
Everything has to
be caused by something, and infection is no different. There are six basic
causative agents of infection.
Bacteria are single-celled plant-like microbes that act in a variety of ways to
produce symptoms of infection. For example, they produce toxins or poisons in
the body. Although bacteria reproduce inside the body, they do so outside the
human cell as well and thus can be killed or inhibited by antibiotics. Strep
throat, tuberculosis, pneumonia, gonorrhea, cholera and whooping cough are
examples of infections caused by bacteria.
Viruses, chains of nucleic acid surrounded by a protein coat, are the most
primitive form of life and cannot reproduce on their own. However, when they
invade the human cell, they use the cell's reproductive capabilities. They are
the tiniest and toughest of infectious pathogens. Antibiotics have no effect on
viruses, and no drugs exist that can kill them. Some physicians prescribe
antibiotics for patients with influenza to prevent secondary infection, but they
have no effect on the influenza itself. Interferon, a protective
substance made by the body, is one of the main means of attacking viruses. Some
viral infections have been successfully prevented with vaccines. Examples of
infections caused by viruses include the common cold, influenza, measles,
rabies, herpes, smallpox and AIDS.
Protozoa are single-celled parasites that produce toxins and release enzymes
that interrupt the body's ability to function normally. Protozoa-caused
infections, such as malaria, sleeping sickness and amoebic dysentery, generally
are associated with a tropical environment and poor sanitation and are not a
public health problem in the United States. The one major exception is
giardiasis, which can be caught by drinking contaminated water. Hikers and
travelers are especially at risk for getting giardiasis. Protozoan infections
can be treated with drugs.
As the name indicates, parasitic worms are multicelled animals usually shaped
like flat or round worms. They release toxins inside the body and feed off blood
and compete for food with the host. As a result, people infected with worms-for
example, tapeworm disease-often are anemic and
malnourished. Most worm infections can be treated successfully with drugs and
often can be prevented with good hygiene and proper cooking of food. Trichinosis
is an example of a parasitic infection caused by not cooking meat thoroughly.
Fungi are either single-celled or multicelled plant-like organisms that usually
grow on the surface of the skin. They release enzymes that in turn digest cells.
Antifungal topical ointments and drugs can kill fungi. Athlete's foot is an
example of an infection caused by a fungus. Fungi are also allergens and in some
parts of the country cause considerable discomfort.
A microorganism that is often described as somewhere between viruses and
bacteria is rickettsia. It is much larger than a virus, is treated with
antibiotics, and is parasitic in nature (requires living cells for growth).
Rickettsial infection is usually transmitted by insects such as ticks. Common
rickettsial diseases are Rocky Mountain spotted fever and typhus.