Pinworm, a small roundworm that lives as a parasite, most often in the digestive tract of human beings. (Pinworms can also be parasitic in horses, rabbits, and hamsters.) Pinworms are found most often in children, but adults may also become infected. The eggs are swallowed and hatch in the stomach. The larvae pass into the intestines, and the adult pinworms—about one-half inch (13 mm) long—develop in the large intestine. The adult females pass out of the intestine and lay their eggs around the anus, causing an itching sensation.

People may carry pinworm eggs to their mouths or to food on their fingers after scratching themselves. Pinworm eggs may also be drawn into the mouth from the air, if an infected person's clothing or bedclothes are shaken out, thus scattering the eggs. Infection may cause sleeplessness, loss of appetite, nausea, and irritability.

Pinworms can be removed from the body, and symptoms relieved, by proper medication. To control infection, hands should always be washed with soap and water before touching food or the mouth. Clothes, towels, and bedclothes should be boiled.

The human pinworm is Enterobius vermicularis; horse pinworm, Oxyuris equi; rabbit pinworm, Passalurus ambiguus; hamster pinworm, Syphacia obvelata. All are of the class Nematoda.