Onchocerciasis (pronounce [ɒn.kəʊ.sɜːˈkaɪə.sɪs]) or river blindness is the world's second leading infectious cause of blindness. It is caused by Onchocerca volvulus, a nematode that can live for up to fifteen years in the human body. It is transmitted to people through the bite of a black fly. The worms spread throughout the body, and when they die, they cause intense itching and a strong immune system response that can destroy nearby tissue, such as the eye.