1. Adult worms live in the small intestine of people. There, females may produce about 200,000 eggs per day. The eggs are excreted with stool.
2. Only fertilized eggs cause infection.
3. The fertilized eggs develop in the soil. The eggs develop best in moist, warm, shaded soil.
4. People become infected when they swallow Ascaris eggs, often in food that came in contact with soil contaminated with human stool containing fertilized Ascaris eggs.
5. The eggs hatch and release larvae in the intestine.
6. The larvae penetrate the wall of the small intestine and travel through the lymphatic vessels and bloodstream to the lungs.
7. Once inside the lungs, larvae pass into air sacs (alveoli) in the lungs, move up the respiratory tract and into the throat, and are swallowed. When the larvae reach the small intestine, they develop into adult worms.
Image from the Centers for Disease Control and Prevention, Global Health, Division of Parasitic Diseases and Malaria.