Infections occur when the body is invaded by micro-organisms. Better housing,
improved hygiene and the development of drugs and vaccines mean that they are no
longer a major cause of illness and death.
Infections range from minor ailments such as colds and flu to potentially
fatal diseases such as rabies and cholera. They can be localized -- confined to
a small area or particular system -- or generalized, that is, affecting a larger
part of the body.
CAUSES OF INFECTION
Many types of micro-organisms can
cause infections, but the vast majority of serious diseases are caused by
viruses and bacteria.
Viruses, the smallest known type of infectious agent, are unable to exist
outside the cells of another living being. Some viruses -- such as HPV, the
human papilloma, or wart, virus and HIV (human immunodeficiency virus) which
causes AIDS -- affect only humans. Others infect animals, plants and even
Viruses come in all shapes and sizes, but their structure is basically the
same. Each is surrounded by an outer shell of protein and this contains a
particle of genetic material, either DNA or RNA. This allows the virus to
A large number have developed extremely sophisticated methods of
reproduction. The HIV virus, for example, multiplies within the lymphocytes --
white blood cells -- which are actually part of the body's defence system.
Others 'hide' in the cells, for example the herpes virus and hepatitis B, and
so they manage to bypass the immune system completely.
Bacteria are much bigger single-celled organisms, which can, and often do,
exist separately from living creatures; for example, some live in the soil.
BACTERIA IN HUMANS
In human beings, some bacteria enter
cells during an infection while others remain outside. Many bacteria, known as
commensal, co-exist quite happily in the colon and, in fact, help prevent the
growth of other more dangerous bacteria, unless they themselves multiply and get
out of control.
Protozoa -- larger single-celled organisms -- and fungi are other important
sources of infection. For example, malaria -- which affects some 300 million
people worldwide -- is caused by the plasmodium protozoa. Candida, or thrush, is
an extremely common infection, particularly in women, and this is caused by a
type of fungus.
The body deals with an infection
using a remarkable and complex defence mechanism, known as the immune system.
In order to reach the cells at all, an organism must first breach the outer
barrier created by the skin -- the body's first line of defence against disease.
Some infections manage to enter the body directly, for example through the
digestive or respiratory tracts.
Once a micro-organism has invaded the body, it may be consumed by a phagocyte
-- a type of white blood cell which swallows and destroys bacteria and viruses.
Pus is a mixture of dead organisms and phagocytes.
The immune system ahs several other different defence tactics. One is the
production of antibodies -- molecules of protein that travel around the
bloodstream ready to bind onto and disarm micro-organisms, so that phagocytes
can come and destroy them.
Antibodies can also stop organisms
from doing their job by acting as gatekeepers, preventing viruses from entering
cells. Alternatively, they may trigger another defence system which leads to
invading bacterial cells being broken down and destroyed.
Another process, known as cell-mediated immunity, involves the cells of the
lymphatic system, or lymphocytes, which are primed to kill a particular
organism. they may also produce certain substances which help phagocytes to
attack invading organisms.
The time between contracting a
disease and actually developing it is known as the incubation period. The onset
can be rapid -- for instance, bacterial meningitis or cholera can develop in a
matter of hours. Other infections lie dormant for weeks, months or even years.
By and large, bacterial infections
become established quickly and their incubation period is often correspondingly
shorter than that of viruses. This is because bacteria immediately start
reproducing in the tissues, thus creating symptoms straight away.
Viruses only produce symptoms once
they have entered the cells and produced 'second generation' viruses which are
released directly into the bloodstream.
Many micro-organisms show a
preference for a specific organ, though just why this is remains a mystery. For
example, the hepatitis virus lodges in the liver; the pneumococcus virus, which
causes pneumonia, settles in the lungs; and the meningococcus virus, which
causes meningitis, attacks the membranes lining the brain.
HOW THEY WORK
The symptoms of infection are
produced in several different ways. Some micro-organisms, such as diphtheria,
act by producing a toxin, or poison. Others destroy prevent large areas of the
lung from functioning properly.
Other micro-organisms, such as
staphylococcus, can produce disease in any system of the body. The invading
organisms are carried around the bloodstream and may settle in organs which are
far away from the original point of entry. Once settled, an organism may
eventually form an abcess.
Abcesses themselves may produce
toxins, which poison parts of the body. Tetanus toxins, for instance, affect the
nervous system, and cholera produces a toxin which causes diarrhoea.
Ironically, many symptoms of
infection are caused by the body's attempts to get better. For example, the
phlegm which is often one of the main symptoms of pneumonia is part of the
body's natural immune response to fight the infection.
Antibiotics, which are available to
combat a large number of bacterial diseases, have revolutionized the treatment
of infections. They are toxic to bacteria but not to human cells. There are also
several antibiotics available to treat protozoal and fungal infections.
Viral infections are more difficult
to treat with drugs because the drugs need to be able to kill the virus, without
damaging the cells in which the organisms live. Therefore, treatment usually
involves supporting the body until the immune system takes over.