Toenail Fungus (known medically as Onychomycosis) is the most common disease of the nail. Nail fungal infections are typically caused by a fungus that belongs to a group of fungi called dermatophytes, though yeasts and molds also can be responsible for nail fungal infections. An infection occurs when fungi take hold underneath one or more of your nails. They invade your skin through tiny invisible cuts or through a small separation between your nail and nail bed. These microscopic organisms thrive in warm, moist environments such as showers, swimming pools, and the inside of a hot, damp shoe.
A nail fungal infection may begin as a white or yellow spot under the tip of your fingernail or toenail. As the nail fungus spreads deeper into your nail, it may cause your nail to discolor, thicken and develop crumbling edges — an unsightly and potentially painful problem.