Adult worms live in the lumen of the small intestine. Female worms produce approximately 200 000 eggs per day, which are excreted in stool and must incubate in soil for 2 to 3 weeks for an embryo to become infectious. Following ingestion of embryonated eggs, usually from contaminated soil, larvae hatch in the small intestine, penetrate the mucosa, and are transported passively by portal blood to the liver and lungs. After migrating into the airways, larvae ascend through the tracheobronchial tree to the pharynx, are swallowed, and mature into adults in the small intestine. Infection with A lumbricoides is widespread but is most common in the tropics, in areas of poor sanitation, and where human feces are used as fertilizer. If infection is untreated, adult worms can live for 12 to 18 months, resulting in daily fecal excretion of large numbers of ova. Female worms are longer than male worms and can measure 40 cm in length and 6 mm in diameter.

The incubation period (interval between ingestion of eggs and development of egg-laying adults) is approximately 8 weeks.