Onychomycosis is a fungal infection of the toenails or fingernails. This ailment causes fingernails or toenails to thicken, discolor, disfigure, and split. At first glance, onychomycosis appears to be only a cosmetic concern. However, without treatment, the toenails can become abnormally thick, forcing the toe to press against the inside of the shoes, causing pressure, irritation, and pain. If the disease continues to progress without treatment, onychomycosis may interfere with standing, walking, and exercising, and can also lead to bacterial infections and spread to the skin.