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INFECTIOUS DISEASES

Infections are the most common type of disease. Many kinds of bacteria, viruses, and other microorganisms can invade the human body and cause diseases. Disease-causing microorganisms are called pathogens. Pathogens take over some of the body's cells and tissues and use them for their own growth and reproduction. In the process, they damage or destroy the cells and tissues and so produce diseases.

Infectious diseases can be grouped according to the kind of pathogen that causes them. Bacteria and viruses are the most common pathogens. But fungi, protozoans, and worms also can cause infectious invasive diseases.

Bacterial diseases. Bacteria are microscopic, one-celled organisms. They rank among the most widespread of all living things. A single grain of soil may contain more than 100 million bacteria.

Most bacterial diseases result when bacteria multiply rapidly in the living tissue, damaging or killing it. Boils and carbuncles result from the multiplication of bacteria in the skin. Bacterial pneumonia occurs when bacteria invade the lungs and multiply there. Many other serious diseases, including tuberculosis, result from bacterial multiplication.

Viral diseases. Viruses are smaller than bacteria. They are so tiny that scientists can see them only by means of powerful electron microscopes. By itself, a virus seems to be a lifeless particle. But after a virus invades a living cell, it uses materials in the cell to reproduce. As a virus multiplies, it damages or destroys the cell. If a number of cells become infected, a disease results.

Viruses cause many common diseases, including chickenpox, measles, mumps, and rubella (German measles). Viruses are also responsible for influenza and the common cold. In fact, scientists have identified more than 100 different viruses that cause the common cold. Most cases of diarrhea and vomiting result from viral infections. Viruses also cause many serious diseases, including hepatitis, polio, rabies, and AIDS (acquired immunodeficiency syndrome). The virus that causes AIDS destroys the immune system's ability to function properly. As a result, people infected with the virus become susceptible to certain illnesses that do not normally occur or that normally are not serious. Many people with AIDS die from these illnesses.

Other infectious diseases can be caused by fungi, protozoans, and worms that live in or on the human body. These pathogens obtain food by breaking down body tissues or by absorbing digested food from the intestines. These pathogens produce diseases ranging from minor skin infections to life-threatening internal disorders.

Spread of infectious diseases. Most infectious diseases are communicable - that is, they can spread from person to person. Occasionally, an infectious disease becomes highly contagious and sweeps through a community. This condition is called an epidemic. When an epidemic occurs at several places throughout the world at the same time, it is called a pandemic. Such an outbreak took place during the winter of 1918-1919, when influenza swept the world, killing about 20 million people. Some infectious diseases are always present in a particular geographic region. Such diseases are said to be endemic in that region. For example, malaria is endemic throughout much of Africa.

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