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What causes a fungal nail infection?

A fungal nail infection (tinea unguium or onychomycosis) is caused by an overgrowth of fungi on your nail or under it.

They are most often caused by the same fungi that causes athlete’s foot, although there are other fungi that can be responsible.

The fungi can often live on your skin without causing you any harm, but if given the right circumstances (they like warm, moist places), they can multiply and lead to an infection.

The three different types of fungus that cause fungal nail infections are:

  • Dermatophytes, like trichophyton rubrum. This is the fungus that causes most cases of athlete’s foot and it causes almost all toenail fungal infections.
  • Moulds called non-dermatophytes. This is a fungus that grows in soil.
  • Yeasts that are usually present on the human body. An overgrowth of these can be caused by things like using antibiotics, using the birth control pill, having immune system problems or certain illnesses.

There isn’t always a specific reason a fungal nail infection develops, but you are more likely to develop one if you:

  • have a fungal skin infection that then spreads to your nails, such as athlete's foot
  • touch an infected toenail with your fingernail, and spread the infection, or vice versa
  • keep your feet or hands in warm, moist and dark conditions for extended periods of time – like sweaty shoes or gloves
  • have your hands or feet in water a lot – perhaps because of your job, for example, if you are a cleaner or swimming teacher
  • have damaged nails
  • walk around barefoot in places where you can pick up the fungi, like at the swimming pool or in a gym’s changing room
  • have certain other health conditions, like diabetes, psoriasis, poor circulation or a weakened immune system
  • are a smoker
  • live in a hot and humid climate
  • wear fake nails
  • have a nail treatment with unsterilised nail clippers or files
  • are over the age of 65
  • share clothes, shoes or towels with other people.