S. stercoralis is a roundworm that is fairly common in warm, moist areas. In rare cases, it can be found as far north as Canada.
People catch the infection when their skin comes in contact with soil contaminated with the worms.
The tiny worm is barely visible to the naked eye. Young roundworms can move through a person's skin and into the bloodstream to the lungs and airways.
They then move up to the throat, where they are swallowed into the stomach. The worms then move to the small intestine, where they attach to the wall. Later, they produce eggs, which hatch into tiny larvae (immature worms) and pass out of the body.
Unlike other worms, these larvae can reenter the body through the skin around the anus, which allows an infection to grow. Areas where the worms go through the skin can become red and painful.
This infection is uncommon in the United States. Most cases in North America are brought by travelers who have visited or lived in South America or Africa.