Nail fungus is a fungal infection in one or more of your nails. This problem is the result of a fungal infection of the nail plate. It generally appears as patches of white or yellow under the tip of your fingernail or toenail surface and edges of the nails. As the nail fungus spreads deeper into your nail, it may cause your nail to discolor, thicken and develop crumbling edges and cause uncomfortable pain. An infection with nail fungus may be difficult to treat, and it may recur.
Though nail fungus is typically caused by a group of fungi called dermatophytes, it may be caused by yeasts and molds as well. Nail fungus is more frequently found in toenails than in fingernails because the toenails are often confined in a dark, warm, moist environment inside your shoes, where fungi can thrive.