(Strongyloides stercoralis), worm of the phylum Nematoda that is parasitic (parasitism) in the human intestine but is able to live freely and breed in the soil. It is especially common in the moist tropics.

Larvae are passed out of the host's body in the feces. They usually reenter through the skin of the host's feet and are carried by the bloodstream to the region of the lungs. They break through the tracheal passages, migrate into the digestive tract, and become localized in the large intestine, maturing and producing eggs there.

The name threadworm is sometimes also applied to other threadlike nematodes, including the species known as the pinworm (q.v.).