Caused by Enterobius vermicularis, a nematode. Humans are the only known host. People in crowded living conditions and children are at higher risk for infection. The adult worms live in the host's cecum. From there they migrate to the perineum at night to lay eggs. Ingested eggs are hatched in the duodenum and migrate to the large intestine, restarting the cycle. Patients will complain of perineal and perianal itching and irritation at night, which leads to poor sleep. Rarely, urinary tract infection, malabsorption and weight loss, and appendicitis have resulted. The worms are rarely seen, so testing should be done to show the presence of eggs.