Athlete's foot

organisms are called dermatophytes. The infection is also called onychomycosis.It usually appears in the toenails because socks and shoes keep the toenails dark, warm, and moist. This is a perfect place for the fungus to grow. The organisms that make up nail fungus can sometimes spread from one person to another because these organisms can live where the air is often moist and people's feet are bare. This can happen in places like showers, bathrooms, or changing rooms, or it can be passed along when you share a nail file or clippers with someone who has a nail fungus infection. Nail fungus may also spread from one of your nails to other nails.

Anything that damages a nail can make it easier for the fungus to move in, such as: An injury (like banging a fingernail with a hammer)
Tight shoes that pinch the toes