The disorder is caused by infestation with the roundworms:
The first two roundworms affect humans only. The last two types also occur in animals.
Hookworm disease is common in the moist tropics and subtropics. It affects about 1 billion people worldwide. In developing nations, the disease leads to the death of many children by increasing their risk for infections that their bodies would normally fight off.
There is very little risk of getting the disease in the United States because of advances in sanitation and waste control. The important factor in getting the disease is walking where people who have hookworm have made feces.
The larvae (immature form of the worm) get into the skin. The larvae move to the lungs via the bloodstream and enter the airways. The worms are about 1/2 inch long.
After traveling up the windpipe, the larva are swallowed. After the larvae are swallowed, they infect the small intestine. They develop into adult worms and live there for 1 or more years. Adult worms and larvae are released in the feces.