Answer: Strongyloides stercoralis larvae. This is a potentially serious infection, and can result in hyperinfection and death in immunocompromised hosts. It is endemic in regions of the world where human feces contaminates the soil, including some parts of the Southeastern United States where sanitation and waste treatment is inadquate.
Infection is acquired when infectious L3 larvae in soil penetrate intact human skin and enter the circulatory system to eventually reach the intestine.
Refer to the CDC DPDx web site for more information on the complicated life cycle of Strongyloides stercoralis.