Onchocerciacis, otherwise known as River Blindness, is caused by the intrusion of parasitic worms (Onchocera volvulus, for all you fans of latin genera) into the body. The worms are capable of surviving for fourteen years within a host, during which time they inject millions of microscopic larvae into the bloodstream.
It is the existence of the microscopic larvae that gives rise to the symptoms associated with River Blindness, and the subsequent transfer of the disease to others. Larvae migration relies upon the actions of the Blackfly--the flies (whose role here as a vector is reminiscent of the mosquito's Yellow Fever days) transfer the larvae by first ingesting them from the host during a bloodmeal and then transferring them to a different individual at a later feeding.
Typically, symptoms will manifest roughly three years after introduction of the worm. The most common include visual impairment (hence the name), depigmentation of the skin, and lymphadenitis (swelling of lymph nodes and vessels; also occurrs in elephantiasis).
River Blindness is most prevalent in Africa--nearly all people infected with the disease reside there (some 99 percent.) Many fertile river areas go abandoned for fear of infection by the high blackfly populations therein.