Hookworms are the second most common intestinal roundworms in humans worldwide, with an estimated half a billion people infected at any one time.
"There are two species that are human pathogens; Ancylostoma duodenale which is found in Africa, India, Europe and China, and Necator americanus, which is found in the Americas. However, many areas are endemic for both species. You get hookworm in areas of unsanitary conditions where people defecate on the ground and the climate is favorable. Warm, moist climates and sandy soil are the environments hookworms thrive in. Infective larvae can survive up to a month in the soil under ideal conditions. People get infected by walking barefoot over contaminated soil where the hookworm larvae can penetrate the skin. It is a particular problem in farmers in developing nations worldwide. Also, small children get it by sitting bare-butt on the ground that has hookworm."