Dana Jackman, age 12, of Austin, Texas,for his question:

ARE ALL PARASITES HARMFUL?

A parasite is a plant or an animal that feeds and lives on or in another plant or animal. The plants or animals on which parasites feed are called hosts. Parasites have varying effects on the body of their hosts but experts believe that most of them cause little or no harm to their hosts.

Some authorities point out that all animals are parasites because they must rely on other living things for food. But in a stricter sense, parasites usually live on plants and animals bigger than they are. And they only feed on small amounts of the host's tissue or food at a time.

One type of amoeba lives in human intestines. It feeds on partly digested food and other intestinal parasites without causing any obvious ill effects.

Other types of parasites may cause great harm. For example, the protozoans or one celled animals that cause malaria are parasites in the red blood cells of human beings.

Many protozoans are parasites. For example, one type of amoeba destroys the lining of the intestines of humans. This produces the painful disease called amoebic dysentery.

Other protozoans may invade the blood of mammals and cause diseases such as malaria and Texas cattle fever. Blood sucking insects and ticks pick up parasites from infected animals and pass them on to other animals and human beings.

Parasitic flatworms and roundworms cause serious damage and often kill their hosts. One group of flatworms, called flukes, live in the intestines, liver, lungs or blood of animals.

Another group, the tapeworms, mature in the intestines of animals. They attach themselves to the intestinal wall with suckers or hooks. The tapeworms then absorb digested food,    depriving the host of nourishment. Hookworms are the most harmful group of roundworms.

  Parasitic insects, ticks and mites usually attack the skin. Their bites are irritating, but the diseases they spread are far more serious. Certain ticks transmit Rocky mountain spotted fever to people. One type of mosquito spreads yellow fever and another carries malaria. The tsetse fly transmits African sleeping sickness. People may get typhus from a body louse.

Insects, ticks and mites may be parasitic only during particular periods of life. As an example, only adult fleas are parasites.

Plant parasites cause many serious diseases in plants, animals and people.

Parasitic fungi cause wheat and bean rust, potato and tomato blight, apple scab and downy mildew of grapes. Fungi also can cause lumpy jaw, a disease that injures the jaws of cattle and hogs. Ringworm is a fungus infection in human beings.

Experts estimate that parasites destroy about $3 billion worth of crops in the United States each year.