Onychomycosis, also known as tinea unguium, is a fungal infection of the nail. This condition may affect toenails or fingernails, but toenail infections are particularly common. Treatment may be based on the signs. Treatment may be with the medication terbinafine. It occurs in about 10 percent of the adult population. It is the most common disease of the nails and constitutes about half of all nail abnormalities.