Infection and Infestation

The human body provides a suitable environment for the growth of many types of micro-organisms, including bacteria, viruses, fungi, yeasts, and protozoa. It may also become the host for animal parasites such as insects, worms, and flukes.

Micro-organisms (microbes) exist all around us and can be transmitted from person to person in many ways: direct contact, inhalation of infected air, and consumption of contaminated food or water. Not all micro-organisms cause disease; many types of bacteria exist on the skin surface or in the bowel without causing ill effects, while others cannot live either in or on the body.

Normally the immune system protects the body from infection. Invading microbes are killed before they can multiply in sufficient numbers to cause serious disease.





Types of Infecting Organism





Some bacteria are aerobic — that is, they require oxygen — and therefore are more likely to infect surface areas such as the skin or respiratory tract. Others are anaerobic and multiply in oxygen-free surroundings such as the bowel or deep puncture wounds. Bacteria can cause symptoms of disease in two principal ways : first, by releasing toxins that harm body cells; second, by provoking an inflammatory response in the infected tissues.





Viruses are smaller than bacteria and consist simply of a core of genetic material surrounded by a protein coat. A virus can multiply only in a living cell by using the host tissue's replicating material.

Protozoa are single celled parasites and are slightly bigger than bacteria. Many protozoa live in the human intestine and are harmless. However, some types cause malaria, sleeping sickness and dysentery.





Infestations



Invasion by parasites that live on the body (such as lice) or in the body (such as tape worms) are known as infestation. Since the body lacks strong natural defenses against infestation, antiparasitic treatment is necessary. Infestation is often associated with tropical climates and poor standards of hygiene.





What Can Go Wrong





Infectious diseases occur when the body is invaded by microbes. This may be caused by the body having little or no natural immunity to the invading organism, or the number of invading microbes being too great for the body's immune system to overcome. Serious infections can occur when the immune system does not function properly, as in the malnourished or the elderly, or when a disease that weakens or destroys the immune system, as occurs in AIDS (acquired immune deficiency syndrome).



Infections (such as childhood infectious diseases or those with flu-like symptoms) can cause generalized illness, or they may affect a specific part of the body (as in wound infections). Some parts are more susceptible to infection than others—respiratory tract infections are relatively common, whereas bone and muscle infections are rare.





Some symptoms are the result of damage to body tissues by the infection, or by toxins released by the microbes. In other cases, they result from the body's defense mechanisms.

Most bacterial and viral infections cause fever; and bacterial infections may also cause pus formation and inflammation in the affected area.





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