Onchocerciasis, also known as river blindness, is caused by the nematode Onchocerca volvulus and can result in dermatitis, subcutaneous nodules, lymphadenitis, and visual impairment, including blindness. The disease is transmitted by the bite of female Simulium flies (black flies) that bite by day and are found near rapidly flowing rivers and streams. It affects over 17 million people in 25 nations in Africa.