Ascaris lumbricoides is a parasitic roundworm that infects the small intestine of humans. The life cycle of A. lumbricoides begins when eggs are excreted from an infected human host and ingested by another human. Inside the new human host, the eggs hatch into larvae, develop through multiple stages in the lungs and intestines, and mature into adult worms that reside in the small intestine where they reproduce sexually and produce eggs to complete the cycle.