The pinworm, or Enterobius vermicularis, is a small parasitic worm that can live inside the human lower intestine. Only 3 mm to 10 mm long, it causes an intense itching in the area of an infected person's anus, especially at night. Pinworms usually infect people as a result of overcrowding in schools and family groupings or due to unsanitary living conditions and poor food preparation. However, they usually don't indicate poor personal hygiene. They can be commonly found in institutional settings such as dormitories and mental health hospitals or homes.

Pinworms mainly infest children 5 to 10 years old, most of whom have no symptoms apart from itching around the anus, which may then become raw from scratching. Pinworms are uncommon in children less than 2 years old. In girls, pinworm infection can cause vaginal itching and irritation (vaginitis). In severe cases, weight loss, restlessness, irritability, and loss of appetite may occur. In a few rare cases, attacks of appendicitis may have been caused by pinworms blocking the appendix.