Richard Mayes Jr., age 13, of Hutchinson, Kan., for his question:
WHAT IS A PARASITE?
A parasite is a plant or an animal that feeds and lives on or in another plant or animal. The plants and animals on which the parasites feed are called the hosts.
Some experts on the subject have pointed 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. They only feed on small amounts of the host's tissue or food at a time.
Parasites have varying effects on the bodies of their hosts. Authorities believe that most parasites cause little or no harm to their host. As an example, one type of amoeba lives in human intestines. It lives on partly digested food and other intestinal parasites without causing any obvious ill effects.
Other types of parasites may cause great harm. An example of this is found in the one celled animals called protozoans that cause malaria. They are parasites that live in the red blood cells of human beings.
Many protozoans are parasites. One type of amoeba destroys the lining of the intestines in humans. This produces the painful disease called amebic dysentery.
Other protozoans may invade the blood of mammals and cause diseases such as Texas cattle fever. Blood sucking insects and ticks can pick up parasites from infected animals and then pass them on to other animals and human beings.
Parasitic insects, tics and mites usually attack the skin. Their bites are irritating but the diseases they spread can be far more serious. Certain ticks can transmit Rocky Mountain spotted fever to people. One type of mosquito spreads yellow fever and another carries malaria.
Plant parasites may cause many serious diseases in plants, animals and people. Experts say that plant parasites destroy more than $3 billion worth of crops in the United States each year.
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 walls with suckers or hooks. The tapeworms then absorb digested food, depriving the .host of nourishment.
Hookworms are the most harmful group of roundworms. They live in intestines and feed on the blood of their hosts.
Insects, ticks and mites may be parasitic only during particular periods of life. As an example, only adult fleas are parasites. Red bugs and screwworms, a type of fly, are parasites only in their infant or larval stage.
Some animal parasites live on plants and may kill them. Aphids or plant lice, scale insects and threadworm are examples of this.
Mistletoe, a parasite of forest trees, is called a partial parasite because it makes some of its own food.