Parasite, an organism that lives in or on another organism, called the host, from which it obtains its food. An external parasite is qne that lives on the surface of the host and sucks nourishment from the host's body. Fleas and leeches are examples of external parasites. If the parasite is diseased, the host also may be infected. Malaria and typhus fever are transmitted in this way. An internal parasite is one that lives inside the host. Examples are stomach and intestinal worms. The parasite may cause illness by inflaming or clogging an organ, by robbing the host of nourishment, or by secreting poisons.

A parasite may have more than one host. The liver fluke, for example, spends its immature life in the pond snail (called the intermediate, or secondary, host). and its adult life in cattle and sheep (the definitive, or primary, host). It leaves the snail in larval form and attaches itself to aquatic vegetation, which is then eaten by the host.

The most abundant parasites are the viruses and certain types of bacteria. They cause a number of diseases in animals and humans; they are less frequently harmful to plants. The fungi include many parasitic species. Some cause rust, smut, blight, and rot in crops. Others cause ringworm and athlete's foot in humans. Protozoan parasites include the amoeba that causes dysentery. A few flowering plants are sap-sucking parasites on other plants. Examples are witchweed, dodder, and mistletoe.

Virtually every phylum (major group) of animals contains a variety of parasitic species. They attack plants, other animals, and humans. The hookworm, tapeworm, and pinworm cause wasting diseases. Some insects, ticks, and mites spread such diseases as malaria and bubonic plague.