Commonly called "river blindness," onchocerciasis is caused by a parasitic worm, which is spread in the human bloodstream through bites from blackflies and buffalo gnats found in parts of Africa, South America, and Central America. The worm's offspring cause inflammation, bleeding, and other problems in the eye. Without a 15-year regimen of annual doses of Mectizan, blindness will result. (Information supplied by ORBIS International. )