Ancylostomiasis, or hookworm disease, is caused by hookworm infection and is characterized by progressive anemia. In 1989, it was estimated that perhaps as many as one billion people, most of them living in tropical and subtropical regions, are afflicted to some extent with hookworm infection, although it is not known how many thus infected can be said to be victims of hookworm disease. It is difficult to define the difference between hookworm infection and hookworm disease because a host whose diet contains adequate amounts of iron may sustain a worm burden without debilitating consequences that would render a malnourished person anemic. A person exhibiting signs of the anemia associated with hookworm infestation, therefore, may be said to have hookworm disease regardless of the number of parasites present. Hookworm disease does not appear on the short list of major causes of death in developing countries, but it should be regarded as an important contributing factor in millions of deaths annually and as a source in its own right of widespread human suffering.

Two species of intestinal nematode, Ancylostoma duodenale and Necator americanus, are the parasites that cause ancylostomiasis. Although they apparently cause the same disease, there are important differences between the two species. A. duodenale is slightly larger, sickle-shaped, with hooks or teeth; N. americanus is smaller, “S” shaped, with shell-like semilunar cutting plates instead of teeth. Despite being named the “American killer,” N. americanus is less pathogenic than A. duodenale, as measured by comparative blood loss. A. duodenale has a higher reproductive rate and a shorter life-span. It is also able to infect the host in more ways than can N. americanus.