The life cycle of the threadworm is more complicated than that of most worms.
1. Early-stage Strongyloideslarvae (called rhabditiform larvae) are excreted in stool into the soil.
2. There, these larvae can become adults that live and mate in the environment (free-living adults), or they can become larvae that penetrate human skin and cause infection (called filariform larvae).
3. Free-living adult worms mate, and the females produce eggs.
4. Larvae hatch from the eggs.
5. These larvae can develop into free-living adults or filariform larvae.
6. The filariform larvae penetrate people's skin.
7. The larvae migrate by various routes to the small intestine, where they mature into adults.
8. In the small intestine, the adult female worms produce eggs.
9. The eggs hatch into larvae. Most of the larvae are excreted in stool.
10. Some larvae become filariform larvae in the large intestine and re-infect the person. They either penetrate the wall of the intestine or are excreted in stool and penetrate the skin around the anus or the skin of the buttocks or thighs. In both cases, they follow the usual cycle of infection (going to the lungs and throat and being swallowed).
Image from the Centers for Disease Control and Prevention, Global Health, Division of Parasitic Diseases and Malaria.