Pinworm infection is caused by Enterobius vermicularis, a nematode that affects people of all ages and socioeconomic classes. Humans are the only known host. It is the most prevalent nematode in the United States, and it affects more than 30% of children worldwide. People in crowded living conditions and children are at higher risk for infection. Significant risk factors in developing nations include playing on the floor, nail biting, and not washing hands before eating.

The adult worms live in the host's cecum. From there they migrate to the perineum at night to lay up to 15 000 eggs at one time, which can be viable for up to 2-3 days. The eggs can be spread by their presence on the host's hands, in the bedsheets, or in dust, thus infecting the entire family. Ingested eggs are hatched in the duodenum and migrate to the large intestine, restarting the cycle.