The term “filariasis” refers to several diseases of both humans and animals caused by infection with a specific group of parasitic nematodes called filarial worms (named for the hairlike appearance of the adult form). Those worms that affect humans belong to the Order Filarioidea, Family Dipetalonematidae. They include (1) Wuchereria bancrofti and Brugia malayi, which are common causes of elephantiasis (extreme swelling and skin thickening of the legs, scrotum, labia, or arms) and chyluria (lymph and emulsified fat globules in the urine); (2) Loa loa, the “eye worm”; and (3) Onchocerca volvulus, the cause of onchocerciasis. Depending upon their species, adult filarial worms of both sexes reside in the lymphatic system, subcutaneous tissues, or peritoneal and pleural cavities. Sexual reproduction results in embryos (microfilariae) that enter blood or skin, where they are ingested by a particular intermediate host (certain species of mosquitoes, horse fly, black fly, or other arthropods). The microfilariae develop into larvae in their intermediate hosts and then reenter vertebrate hosts (humans or animals) through bites in the skin made by the intermediate host arthropods. Loa Loa is endemic in West and central Africa, whereas onchocerciasis is found in Mexico, Central America, and West Africa. Discussion of human lymphatic filariasis in the remainder of this entry will be limited to the most prevalent form (90 percent of infections), that caused by W. bancrofti (Sasa 1976; Beaver, Jung, and Cupp 1984; Mak 1987; Manson-Bahr and Bell 1987).