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If one of your nails is thick and looks whitish or yellowish or if it crumbles when you touch it, you probably have what doctors call a fungal infection. It may look bad, but it's not serious. Treatment can keep it from getting worse and stop it from hurting.

It's easy to catch a fungal nail infection, particularly in a toenail. But the sooner you treat it, the easier it is to get rid of the infection.¹

• Fungal infections of your nails don't usually cause serious problems.
• If you don't get treatment for an infected nail, it can spread to other nails.
• Serious infections can damage your nail. This can make it hard for you to walk.
• You don't need to be embarrassed about having a fungal nail infection, no matter how horrible you think it looks. Doctors are used to seeing them.
• Fungal nail infections can be cured.
• You can buy some treatments over the counter. Or your doctor or foot doctor can prescribe others if you need them.
• It might take up to a year to get rid of an infection if it's bad.

You may not like the idea of a fungus living on your skin. But a fungus is simply a tiny organism that causes an infection, just like bacteria or a virus. The fungus is so small that you can't see it.

The type of fungus that gets under your nails lives only on top of your skin. It doesn't go inside your body. You may have heard the infection called ringworm, but it's not caused by a worm.

You can't see the tiny fungi that cause these infections. They live in warm, damp places, such as showers, floors around swimming pools and in locker rooms.

If you have athlete's foot (an itchy infection of the skin on your toes that is also caused by a fungus), it can spread to your nails. Fungal infections often affect the nail on your big toe. If you don't treat the infection, it can spread to other nails on your toes and to your fingernails.

There are lots of things you can do to keep from getting another nail infection. For example, you can wear flip-flops when you use locker rooms at the pool. (See How to keep from getting a fungal nail infection for more tips.)

See how much you know about nail infections by taking our quiz.

Your nails start under your skin, just below the cuticle (the thin layer of skin at the base of the nail). They are mainly made of a tough protein called keratin. (Keratin is part of what makes up your hair, skin and nails.) Underneath each nail is a fleshy area that doctors call the nail bed.

The fungi that cause nail infections live in places that are warm and damp.² So, if you work in a kitchen, for example, where your hands are often wet, it's easy for a fungus to get into the soft skin around your nail and work its way underneath the nail. Or you may pick up an infection if you walk around barefoot in damp places, such as around a swimming pool.

• If you get a nail infection, it means that the fungus has gotten under your nail and into the skin underneath it (the nail bed). Once the fungus is there, it feeds on dead skin cells and starts to grow.
• As the fungus grows, it spreads under the nail. This makes more space between the nail and the skin underneath. And this makes the nail look white. But if dirt gets trapped under the nail it can look black.
• Because there's a space between the nail and the skin underneath it, the nail sticks up higher than the other nails. So it's more likely to get bumped and banged as you walk around. When this happens, the nail gets thick.
• If you get a fungal infection, it can be annoying. If you have an infection in your toenail and it gets bad, you may have trouble walking. Or if you have one in your fingernail, you may have a hard time picking up things.

Some people are more likely than others to get a fungal nail infection, either because of the work they do or because there's something about them that gives the infection a way in.

To find out more, see Who's at risk?

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.