Pinworms are parasitic roundworms with a four- to eight-week life cycle. They have the appearance of a thin, white thread that is less than half an inch in length, and may be seen around the anus of infected people or in their fecal matter.

The adult female pinworm travels to the rectum to lay her eggs. She moves at night, depositing her eggs around the rim of the anus, and then she dies.

The human host scratches the anus; itching is triggered by the movement of the female and the presence of her eggs. The eggs are caught under fingernails, transferred to skin, linens and objects in the home, and eventually the eggs may be carried to the mouth.

Pinworm eggs take six hours to mature. When the eggs are swallowed, they hatch in the digestive tract of a human (animals are not considered a source of infection); when they hatch on the surface of the skin, the larvae make their way into the nearest orifice (the vagina or the anus).

Pinworms begin their lives in the small intestine. The larvae migrate to the large intestine and attach to the intestinal wall.

Adult worms live in the large intestine or colon. It is suspected that they consume human fecal matter as a source of food.

Pinworms reproduce through sexual contact; pregnant females can lay 10,000 to 15,000 eggs. The time from initial ingestion of eggs to the first incidence of adult egg-laying takes about a month.