A roundworm infection is caused by Ascaris lumbricoides, an intestinal roundworm. It is the largest intestinal parasite in humans. An estimated 1 billion people are infected worldwide.

What is going on in the body?

A person is infected by eating the eggs of the roundworm. In the small intestine, larvae hatch from the eggs and penetrate the intestine. They travel in the blood stream to the liver and then the lungs. The larvae rise up the airways and into the mouth. They are swallowed and proceed to mature into adults in the small intestine.

The adult worms live in the small intestine, where the females produce about 200,000 eggs a day. These eggs are excreted in the stool. After incubating for 2 to 3 weeks outside the human body, the eggs become infectious.

What are the causes and risks of the infection?

Roundworm infections are widely found in tropical and humid areas, including the southeastern United States. Infection usually occurs through contact with contaminated soil. The cause is often a lack of sanitation, or the use of human manure as fertilizer. A person can also be infected by eating foods that are contaminated with the cysts of the roundworm.