Roundworms (also known as Ascaris lumbricoides) belong to a large group of worms called nematodes. Roundworms are common human parasites that live in the intestine (guts).

Roundworm is rare in developed countries. It is most common in warm, moist climates, so the most likely time to contract roundworms is when traveling abroad.

A female roundworm living in a persons intestines lays eggs that are passed out in feces. The eggs can survive for over three years in suitable conditions, such as moist soil. The eggs can get into food or water, or onto a persons hands, through poor sanitation, using human excrement as a fertilizer, or inadequate personal hygiene. The infection spreads in humans by eating food or drinking water containing worm eggs.

Eggs that have been eaten hatch in the small intestine and become larvae. The larvae penetrate the bowel lining and are carried in the blood to the lungs, where they grow to about the size of a pinhead. They then migrate up the air passages to back of the throat and are swallowed.

When they return to the small intestine, the larvae mature into adult worms and start to reproduce. Mature worms can live for up to 2 years in the intestine.