Pinworms (Enterobious vermicularis) may be the most prevalent worm parasite in the United States and Europe. It is relatively rare in the tropics. Humans are the only host, and transmission is maintained through a fecal-oral route or, uncommonly, by inhalation of the eggs. Also called seatworms or threadworms, they are commonly found amongst children. School children get pinworms easily because they spend a lot of time with others who may have pinworms, which are highly contagious. Studies of caucasian children in the USA and Canada have shown incidences of infection from 30-80%, with similar levels in Europe. The eggs are so light that they can even float in the air. Pinworm eggs are swallowed and hatch in the small intestine. They then move on to the large intestine where they find a warm, dark place to attach and live. After about 2 weeks, the female pinworms move to the end of the large intestine and then out of the body (usually at night) to lay their eggs around the anus, which causes itching. The eggs are then spread by scratching where they contaminate underwear, nightclothes, bed clothes, hands, and anything the contaminated hands may touch. At night, the adult worms can sometimes be seen directly in bedclothes or around the anal area.

If pinworms are suspected, transparent adhesive tape (often called the "scotch tape test") or a pinworm paddle (supplied by your health care provider) are applied to the anal region. The eggs become glued to the sticky tape or paddle and are identified by examination under a microscope. Because bathing or having a bowel movement may remove eggs, the test should be done as soon as you wake in the morning. Three specimens should be taken on consecutive days before pinworm infection is ruled out. Since scratching of the anal region is common, samples taken from under the fingernails may also contain eggs. Eggs are rarely found during lab examinations of stool.

Pinworm infection is unlikely to cause other conditions. There are cases where the parasite has been suspected as a cause of appendicitis, and pregnant female worms have been known to migrate up the vagina and fallopian tubes and into the peritoneal cavity, where they become encapsulated. Recurrent urinary tract infections have been attributed to ectopic pinworm infections.