Nail fungus is made up of tiny organisms that can infect fingernails and toenails. These organisms are called dermatophytes. Nail fungus is very common. More than 35 million people in the United States have it under their nails. A nail fungus infection is also called onychomycosis (ON-i-ko-my-KO-sis).

The nail fungus moves in under the nail. The nail provides a safe place for the fungus and protects it while it grows. That's why it's so hard to reach and stop nail fungus.

Most often, nail fungus appears in the toenails. This is 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 shower stalls, bathrooms, or locker 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.