A scanning electron microscope image of an adult black fly with the parasite Onchocerca volvulus (shown in red) emerging from the insect's antenna. O. volvulus is a nematode that causes river blindness, the world's second leading infectious cause of blindness. About 18 million people are currently infected with this parasite. Approximately 300,000 have been irreversibly blinded by it.Image credit: Agricultural Research Service