Onchocerciasis is a parasitic infection brings about skin lesions and severe eye damage and that can lead to irreversible blindness. Fertility-change associated genetic selection in parasite populations has now been found. Although the results do not prove with absolute certainty that the parasite Onchocerca volvulus is steadily becoming resistance to ivermectin, the only treatment available, they do show that certain parasites are more sensitive to treatment than others.