The pinworm (Genus Enterobius) is a parasitic roundworm of the phylum Nematoda. The best known is the human pinworm, also known as the threadworm Enterobius vermicularis. It is found worldwide and causes the common infection enterobiasis in humans. The adult pinworm male is 1–4 mm in length, while the adult female is 8–13 mm and possess the long, pin-shaped posterior end for which the worm is named.

The pinworm lives in the lower part of the small intestine, and the upper part of the colon. After mating the male dies. The female migrates to the anus, and emerges (usually during the night) to deposit 10000 to 20000 eggs in the perianal area (around the anus). It secretes a substance that causes a very strong itching sensation, inciting the host to scratch himself and thus involuntatrily to pick up the eggs with the fingers. Once ingested orally, the larvae hatch and grow to maturity in 30-45 days.