A fungal nail infection occurs when a fungus invades a fingernail, a toenail, or the skin under the nail (nail bed). The infected nail usually turns yellow or white, and it may crumble and split. Bits of skin and nail may build up under the nail. The nail may also get thick, and it may separate from the skin.

Most infections are caused by the same fungus that causes athlete's foot. You can get infected by walking barefoot in public showers or pools or from sharing personal items, such as towels or nail clippers. Some people are more likely to get fungal infections than other people are.

Fungal nail infections usually do not get better without treatment. Even with treatment, these infections often come back.