Health A-Z

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What Is It?

Toenail fungus is a condition that disfigures and sometimes destroys the nail. It is also called onychomycosis

Toenail fungus can be caused by several different types of fungi. Fungi are microscopic organisms related to mold and mildew.

These fungi thrive in the dark, moist and stuffy environment inside shoes. As they grow, fungi feed on keratin. Keratin is the protein that makes up the hard surface of the toenails.

Factors that increase the risk of developing toenail fungus include:

  • Wearing tight-fitting shoes or tight hosiery

  • Practicing poor foot hygiene

  • Wearing layers of toenail polish, which doesn't allow the nail to breathe

  • Being a military personnel, athlete or miner. This is because toenail fungi may spread from foot to foot on the floors of showers and locker rooms.

  • Having a chronic illnesses, such as diabetes or HIV

  • Having a circulatory problem that decreases blood flow to the toes

However, many people with toenail fungus have no clear risk factors.

Toenails on the big toe and little toe are the most likely to develop a toenail fungus. This may be partly because the big toe and little toe are constantly exposed to friction from the sides of shoes.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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