Onchocerciasis is an infection caused by the parasitic worm Onchocerca volvulus, spread by the bite of an infected Simulium blackfly. It is also called river blindness because the fly that transmits infection breeds in rapidly flowing streams, mostly near remote rural villages, and the disease that is caused by O. volvulus following repeated bites from infected blackflies can lead to blindness. Persons with heavy infections will usually have one or more of three conditions: skin rash (usually itchy), eye disease, or nodules under the skin. The Global Burden of Disease Study estimated that in 2017 there were at least 20.9 million people infected worldwide, of which 14.6 million had skin disease and 1.15 million had vision loss. More than 99% of infected persons live in Africa; the remainder live in foci in Yemen and Latin America. Onchocerciasis is commonly treated with an oral medicine called ivermectin