With the exception of the roundworm that causes trichinosis, mature roundworms eventually end up or live in human intestines and cause a variety of health problems. We discuss roundworms that infect human intestines only in this fact sheet. In the United States, it is the most common of all parasitic roundworm infections, affecting up to one-third of the country's children. Because pinworm infection is spread mainly by children, it is found most often in family groups, day-care centers, schools, and camps. Pinworms are small, threadlike roundworms found primarily in the colon and rectum.
Pinworms enter the body when eggs are swallowed.
The female pinworm expels thousands of eggs into the environment. Because the eggs are moist and a bit resistant to drying, they may be able to infect someone for several days after being distributed in dust. They can cling to the fingers of children. The eggs may be scattered into the air from bed linen and clothing, and can cling to doorknobs, furniture, tubs and faucets, and even food. Although a person may have no symptoms over a long period, episodes of infection may return repeatedly.