Life cycle of Ascaris lumbricoides, the cause of human ascariasis
Adult Ascaris lumbricoides worms (1) live in the lumen of the small intestine. A female may produce 200,000 eggs each day, which are passed with the feces (2) of the host. Ingested unfertilized eggs are not infective, but fertile eggs begin to develop and become infective after 18 days to several weeks (3), depending on environmental conditions (an optimal environment being moist, warm, shaded soil). After infective eggs are swallowed (4), the larvae hatch (5), invade the intestinal mucosa, and are carried via first the portal and then the systemic circulation to the lungs (6). The larvae mature further in the lungs for 10 to 14 days, then penetrate the alveolar walls, ascend the bronchial tree to the throat, and are swallowed (7). Upon reaching the small intestine, they develop into adult worms (1). Between two and three months are required from ingestion of infective eggs to oviposition (egg-laying) by the adult female. Adult worms can live one to two years.
From Centers for Disease Control Parasites and Health website