Computer illustration of the parasitic hookworm Ancylostoma duodenale in the small intestine. This is one of the commonest causes of hookworm infestation in humans, particularly in sub-tropical regions such as North Africa and India. The worm's head contains several tooth-like structures with which it clings to the bowel of it's host. Hookworms also infect dogs and cats. Human infestation begins with larvae penetrating the skin, usually by burrowing into the feet of a person and entering the bloodstream. The larvae then migrate throughout the body, particularly the small intestine.