Nail fungus is a skin disease that causes infections in the nails. Nail fungus manifests itself with symptoms such as thickening, yellowing, deformity and brittleness of the toenails. Nail fungus is usually more common in toenails, as toenails can cause fungus to grow in a humid and warm environment. While nail fungus is usually harmless, left untreated it can progress and cause pain, nail loss and other skin problems. Treatment of nail fungus may include antifungal medications, special nail polishes, and nail care methods. For the prevention of nail fungus, foot hygiene, choosing the right shoes and keeping the feet moist are important.