In the United States, the most well-known skin-penetrating parasitic worm, called a nematode, is the hookworm. But globally, it is estimated that over 600 million people are infected with the skin-penetrating threadworm, also known as Strongyloides stercoralis. This species is found mostly in tropical and subtropical regions with poor sanitation infrastructure. Skin-penetrating nematodes are excreted in the feces of an infected host, and then enter the ground to wait for a new host. When they infect a new host, they can cause serious illnesses.