Hookworms are parasitic intestinal nematodes, several of which are zoonotic. In their normal hosts, hookworms may enter the body either by ingestion or through the skin. Larvae that penetrate the skin travel through various organs, including the respiratory tract, before entering the intestines and developing into mature hookworms. Hookworms can cause anemia, abdominal pain and diarrhea when they reside in the intestines, or respiratory, dermatologic and other signs during their migration through the body. Young individuals tend to be affected more severely. In cattle, infections may lead to severe disease and pronounced weight loss, with as few as 50 adult worms causing significant anemia in calves. Hookworm disease in cats and dogs can result in anemia, and infections of neonatal pups may prove fatal, even with as few as 50-100 worms present.

Animal hookworm larvae can penetrate the human epidermis, but most species cannot readily enter the dermis, and remain trapped in the skin. These larvae migrate extensively within the skin for a time, resulting in a highly pruritic but self-limited disease called cutaneous larva migrans. One species carried by dogs and cats is increasingly recognized as an intestinal parasite of humans: Ancylostoma ceylanicum has been found in 6-23% of patent human hookworm infections in some parts of Asia. A. caninum also migrates occasionally to the intestines, but usually as a single worm. While one hookworm is unlikely to cause significant blood loss, its presence may result in a painful intestinal disorder called eosinophilic enteritis.