Nail Fungus: An infection of nail fungus occurs when fungi infect one or more of your nails. 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. These infections usually develop on nails continually exposed to warm, moist environments, such as sweaty shoes or shower floors. Na