Toenail fungus is a fungal infection that affects the toenails, causing toenails to turn yellow and crumbly from fungi that squeeze under them. If left untreated, the fungi dig in deeper and may spread to other toenails, causing them to thicken and become even more unsightly. More than one fungi can affect the toenails, with the most common being dermatophytes, which feed off of keratin found in your toe and fingernails.
A fungal infection of the toenail usually occurs due to small cuts in the skin or a small separation in the toenail and the nail bed. Once present in a toenail, the infection can spread quickly. The foot is prone to fungal infections because shoes are dark, moist environments. The infection also thrives from prolonged exposure to warm, moist environments such as pools or showers.