With the exception of Trichinella spiralis , which causes trichinosis, mature roundworms eventually end up or reside in the intestine and cause a variety of health problems. Nematodes that do not infect human intestines are not discussed in this fact sheet. The pinworm is the most common roundworm parasite in temperate climates even in areas with high levels of sanitation. 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 most prevalent 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 moderately resistant to drying, they may remain infectious for several days after being disseminated in dust and 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 an individual may have no symptoms over a long period, there may be repeated episodes of infection.