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5 minute read · Last reviewed 22 October 2026

Fungal nail infection: what it is and what it looks like

A fungal nail infection is a fungus growing in or under the nail, usually a toenail. It starts as a white or yellow patch at the tip or edge of the nail, then spreads back towards the base, making the nail thick, discoloured and crumbly over months. It is not serious, but it rarely clears on its own and is slow to treat, so the earlier it is identified the better.

Key facts

QuestionAnswer
Early signA white or yellow patch at the tip or side of the nail
Later signsThick, yellow-brown, crumbly nail that lifts from the nail bed
Usual causeDermatophyte fungi, the same family that causes athlete’s foot
Goes away on its own?Almost never; it usually spreads slowly
How to confirmA nail clipping test from a GP or podiatrist, results in 2 to 3 weeks

What does a fungal nail infection look like?

A fungal nail infection changes slowly, which is why it is often ignored for a year or more before anyone seeks help. The typical progression:

  • Early: a small white, yellow or cream patch at the free edge or down one side of the nail. The nail is otherwise normal. Easy to mistake for a knock or a bit of dry nail.
  • Established: the patch spreads back towards the cuticle. The nail thickens, turns yellow-brown, and the end begins to crumble or lift away from the nail bed. Debris collects underneath. The nail may be hard to cut.
  • Advanced: the whole nail is thick, misshapen, crumbly and discoloured, and may lift off entirely. The skin around it can become sore. Neighbouring nails are often infected by this stage.

Fungal nail infections are rarely painful early on. Pain comes later, when the thickened nail presses into the shoe or the lifted nail catches.

What are the four types of fungal nail infection?

Podiatrists and dermatologists describe four patterns of fungal nail infection, which matter because they affect which treatment works:

  1. From the tip (distal subungual). By far the most common. The fungus enters under the free edge and grows back towards the base. Yellow-brown thickening and crumbling at the tip.
  2. White patches on the surface (white superficial). Chalky white spots or streaks on the top of the nail that can be scraped away. Milder and more responsive to paint-on treatments, because the fungus is on the surface rather than under the nail.
  3. From the base (proximal subungual). Discolouration starting under the cuticle and growing out with the nail. Uncommon, and more often seen in people with a weakened immune system, so it is worth a check-up.
  4. The whole nail (total dystrophic). The end stage of any of the above: the entire nail is thick, crumbly and discoloured. Hardest to treat and most likely to need tablets or nail removal.

What causes a fungal nail infection?

The usual culprits are dermatophyte fungi, the same family that causes athlete’s foot and ringworm, which feed on keratin, the protein that nails are made of. Yeasts and moulds account for a smaller share, more often in fingernails.

The fungus needs a way in and a warm, damp place to grow. Risk is higher if you:

  • Have athlete’s foot, which spreads to the nails if untreated.
  • Have hot, sweaty feet in trainers or work boots for long hours.
  • Damage a nail, for example by running or a dropped weight.
  • Are older; nails grow more slowly and circulation is poorer.
  • Have diabetes, psoriasis, poor circulation or a weakened immune system.
  • Share showers, pools or nail tools.

Toenails are infected far more often than fingernails because they are in warm damp shoes, grow more slowly and get knocked more.

What else could a discoloured, thick nail be?

About half of the discoloured, thickened nails that people assume are fungal turn out to be something else. The common alternatives:

  • Psoriasis, which pits and lifts nails and often affects several at once. Look for psoriasis elsewhere on the skin.
  • Injury, from tight shoes, running or a knock, which thickens the nail and can leave it discoloured for a year as it grows out.
  • Bacterial infection, which turns the nail green-black. Needs different treatment.
  • Yellow nail syndrome and other rarer conditions.
  • A dark streak or spot under the nail that does not grow out with the nail must be checked by a doctor, because rarely it is a melanoma.

Because antifungal treatment takes six to twelve months, treating a nail that is not fungal wastes a lot of time. The NHS and NICE both advise confirming the diagnosis with a nail sample before starting tablets, and it is sensible before a long course of paint-on treatment too. A podiatrist or GP takes clippings and scrapings from under the nail and sends them to a laboratory; results take two to three weeks.

Should you treat a fungal nail infection?

Fungal nail infections are not dangerous for most people, and some choose to live with them. Reasons to treat:

  • It spreads, to other nails and sometimes back to the skin as athlete’s foot.
  • It gets harder to treat the longer it is left and the more of the nail is involved.
  • A thick nail becomes painful in shoes and can become ingrown.
  • For people with diabetes or poor circulation, a damaged nail is a route for bacterial infection, which can be serious.
  • Appearance, which is reason enough.

Our guide to fungal nail treatment options compares paints, tablets, laser and nail removal, with what each costs and how well the evidence says each works. For people who cannot take tablets, our guide to laser treatment for fungal nails covers the main alternative.

When to see a podiatrist or GP

See a podiatrist if you want the diagnosis confirmed, the nail thinned so it is comfortable and treatment can reach the fungus, or a treatment the pharmacy does not offer. See your GP if you have diabetes, poor circulation or a weakened immune system, if the infection has spread to several nails, or if pharmacy treatment has failed and you want to discuss tablets. Our guide to fungal nail infection on the NHS explains what each can offer.

Nailfungustreatment.co.uk lists 2,859 podiatry and chiropody clinics across 713 UK towns and cities; 640 say they treat fungal nails. Published starting prices for a fungal nail appointment are typically £48 (checked October 2026). Look for “HCPC” on a listing and check the name on the HCPC register.

Sources

Common questions

What colour is a fungal nail?

Usually yellow, white or brown, sometimes with a greenish tinge. The discolouration most often starts at the tip or side edge of the nail and spreads back towards the base over months. A greenish-black nail is more often a bacterial infection, and a dark streak under the nail needs checking by a doctor.

Is nail fungus the same as a fungal nail infection?

Yes. Nail fungus, fungal nail, fungal nail infection, onychomycosis and tinea unguium all describe the same thing: a fungus growing in or under the nail. 'Nail fungus' is the American term; the NHS says 'fungal nail infection'.

Does a fungal nail infection go away on its own?

Almost never. Unlike a verruca, which the immune system usually clears, a fungal nail infection tends to spread slowly along the nail and to other nails until it is treated. Early treatment is quicker and more likely to work.

How do I know if it's fungus or something else?

You often can't by looking: psoriasis, injury, and bacterial infection all discolour and thicken nails. A podiatrist or GP can send a nail clipping to a laboratory, which confirms fungus in about two to three weeks. It's worth doing before months of treatment.

Is a fungal nail infection contagious?

Mildly. The fungus spreads on damp shared floors, towels and nail tools, and from athlete's foot to the nails. It also spreads from one of your nails to others. Treating athlete's foot, wearing flip-flops in shared showers and not sharing nail clippers or files reduce the risk to you and your household.

What happens if you leave a fungal nail infection untreated?

It does not clear on its own. The nail slowly thickens, crumbles and discolours, the infection can spread to other nails and to the skin as athlete's foot, and a thick nail can become painful in shoes. The longer and more extensive the infection, the longer treatment takes, because the new nail has to grow out from the base.

Can a fungal nail infection spread to other parts of the body?

To other nails and to the surrounding skin, yes: the same fungi cause athlete's foot, and the two often go together. In people with a healthy immune system it does not spread beyond the skin and nails. People with diabetes or a weakened immune system should see a GP or podiatrist, because skin breaks around an infected nail can let bacteria in.

How do you get toenail fungus?

The fungi that cause athlete's foot get under the edge of a nail, usually through a small gap, a knock to the nail or skin that stays damp. Warm, sweaty shoes, communal showers, shared nail tools and an existing athlete's foot infection are the usual routes, and older age, diabetes and poor circulation make it more likely.

Does nail fungus hurt?

Not at first. The infection itself is painless, which is why it is often ignored for years. Pain comes later, when the nail thickens enough to press on the shoe, lifts away from the nail bed, or cracks and lets bacteria in.

Is nail fungus dangerous?

For most people, no: it is a nuisance that slowly spreads to other nails and the skin. It becomes a risk for people with diabetes, poor circulation or a weakened immune system, in whom a cracked or lifted nail can let in a bacterial infection that spreads to the foot. In those groups, a fungal nail should be treated and the foot checked regularly.

What should you do if a toenail falls off from fungus?

Keep the exposed nail bed clean and covered with a non-stick dressing until it hardens, wear roomy shoes, and see a podiatrist or GP, because the new nail will grow back infected unless the fungus is treated. A nail takes 12 to 18 months to regrow.

What should you not do with nail fungus?

Do not file it down to the quick, cut the skin around it, cover it with nail polish or acrylics during treatment, share clippers or files, walk barefoot in communal wet areas, or stop treatment when the nail looks better but has not fully grown out. Each of those either spreads the fungus or lets it return.

This guide is general information, not medical advice. Speak to a qualified professional about your own situation.

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