A fungal nail infection is a fungus that gets into your fingernail, toenail, or the skin under your nail, usually through a small cut.
The infected toenail usually turns white or yellow. Sometimes it's painful.
Toenails that are exposed to moisture and warmth a lot may be more likely to get infected by a fungus. This can happen from wearing sweaty shoes often.
Toenail fungus usually doesn't need treatment unless it's causing pain. It often comes back after it has cleared up.