A roundworm that causes gastrointestinal infections (primarily in persons from developing nations) and opportunistic infections (in immunosuppressed patients). It may occasionally be life-threatening. In the U.S., S. stercoralis is found mainly in the rural South. The ova hatch in the intestines of the host, and rod-shaped larvae are passed in the stool. In the soil, these may develop into adults and continue their life cycle or may metamorphose into filariform larvae that can infect humans. The filariform larvae enter the skin, pass through the venous system to the lungs, where they migrate upward and are swallowed. A rash or pneumonia may accompany their migration. The larvae mature in the intestine, and ova of the next generation hatch. The rod-shaped larvae may metamorphose into the filariform larvae in the intestine. These may enter the circulation, migrate to the lungs, and begin the cycle again.