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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.


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 bacteria.

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 reproduce.

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.


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.



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.


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