Nail fungus: causes, symptoms, stages and prevention

Published August 1, 2026 · Updated September 15, 2026

What nail fungus is, and where it lives

Nail fungus, medically called onychomycosis (tinea unguium when caused by dermatophytes), is a fungal infection of the nail and the nail bed beneath it. It is extremely common, more so with age, and it is not a sign of poor hygiene. Dermatophytes cause most cases, with yeasts and moulds behind a minority.

The part that matters for understanding treatment is where the infection actually sits: not on the visible surface of the nail, but underneath the hard nail plate, in the nail bed. That is why anything painted only on top of the nail has a physical barrier to get through before it can reach the problem.

Causes and risk factors

Fungus thrives in warm, damp, dark places — exactly what a shoe is. Common routes in include:

• Untreated athlete's foot spreading from the skin to the nail.
• Communal showers, pool decks and gym changing rooms.
• Nail salons with poorly sterilised tools.
• Small injuries or repeated pressure from tight shoes.
• Sweaty feet in non-breathable footwear day after day.

Risk rises with age, with reduced circulation, and with conditions like diabetes or a weakened immune system. Anyone in those groups should treat a nail change as a reason to see a doctor rather than self-treat.

Symptoms by stage

Nail fungus develops slowly, and what you see roughly tracks how far it has spread through the nail plate and bed.

Fingernails grow roughly twice as fast as toenails and spend far less time in warm, damp enclosures, so fingernail cases usually clear sooner. Toenails, especially big toes, grow about 1mm a month — plan on 9 to 12 months for a full grow-out no matter what you use.

How it's diagnosed

Doctors often diagnose nail fungus visually, since the pattern of discolouration and thickening is fairly distinctive. To confirm it — and rule out look-alike conditions — they may take a small nail clipping or scraping and send it for lab testing under a microscope or for culture. This matters because several other conditions can look similar at a glance but need a different approach.

Conditions it's mistaken for

Not every discoloured or thickened nail is fungus. These are the conditions people most often confuse it with:

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Treatment options, briefly

Once fungus is confirmed, the options range from over-the-counter creams and at-home light therapy through prescription lacquers and tablets to in-clinic laser. None of them beat the speed the nail grows — the damaged nail has to grow out no matter what stops the fungus underneath it. Creams struggle to cross the nail plate; oral prescriptions reach the nail bed through the bloodstream but need a doctor and monitoring; light therapy sits in between, reaching the nail bed without a prescription, at a one-off cost.

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Prevention that actually helps

• Dry between your toes after every shower.
• Rotate shoes so each pair dries fully for 24 hours.
• Choose breathable socks and change them daily.
• Wear sandals in communal wet areas — showers, pool decks, gym floors.
• Treat athlete's foot the moment you spot it.
• Keep your own nail clippers and disinfect them, rather than sharing salon tools.

Reinfection: why it comes back

Nail fungus can return if the conditions that caused it in the first place come back — damp shoes, shared showers, untreated athlete's foot on the surrounding skin. Clearing the nail without addressing those habits is one of the most common reasons an infection resurfaces months later. Replacing old, spore-holding footwear and keeping up the drying habits above matters as much as whatever treatment you use.

When to see a doctor

See a doctor or podiatrist rather than self-treating if you have diabetes, poor circulation or a weakened immune system; if the nail is painful; if the skin around it is red, swollen or leaking; if the infection is spreading quickly; if the nail lifted after an injury; or if a dark streak runs lengthways through it. Those situations need supervision and sometimes a different diagnosis altogether.

This page is general information, not medical advice. FungaBeam is a consumer wellness device and is not intended to diagnose, treat, cure or prevent any disease. Speak to a doctor or podiatrist about your own situation, particularly if you have diabetes, circulation problems or worsening symptoms.