The field of medicine that pertains to helminths (worms) capable of disease in people. The public health impact of medical helminths is appreciable. Two billion people are infected by soil-transmitted helminths such as Ascaris, hookworms, and Trichuris trichiura and by schistosomes. Early childhood infections by soil-transmitted helminths delays physical and cognitive development. Other widespread helminthic infections include onchocerciasis, lymphatic filariasis, dracunculiasis (Guinea worm disease), and food-borne Trematode and tapeworm infections. All of these infections cause chronic morbidity and debilitation. Medical helminths need to develop in a parasitized host, and sometimes this involves several disparate hosts. Helminth parasites are more complex than free-living helminths, because they have evolved mechanisms to deal with the different environments of their various hosts and living conditions. They have developed host-finding behaviours, exquisite migration patterns within each host, and the ability to evade the host immune and protective responses.