When a female Simulium blackfly bites an infected person during a blood meal, microfilariae are transferred to from the person the fly. Over the course of one to three weeks, the microfilariae develop inside the blackfly to form infective larvae. These are then passed on to other people when the blackfly takes another blood meal. In the human host, the larvae migrate in the subcutaneous tissue, form nodules and slowly mature into adult worms, completing the cycle.