If your toenail has started looking thick, discoloured, or crumbly, you might already be wondering what’s behind it. Fungal toenail infections are more common than most people realise — and while they’re rarely dangerous, they can be stubborn, uncomfortable, and a source of real self-consciousness.
Understanding what causes a fungal toenail infection is the first step toward treating it effectively and stopping it from coming back. Here, our podiatrists break down exactly how these infections start, who’s most at risk, and what you can do about it.
What Is a Fungal Toenail Infection?
A fungal toenail infection (medically known as onychomycosis) occurs when a fungus takes hold in or under your toenail. The result is a nail that may appear yellow, brown, or white, and often becomes thickened, rough, brittle at the edges, or separated from the nail bed.
Not every discoloured toenail is fungal. Trauma to the nail, psoriasis, and bacterial infections can look similar, which is why a proper diagnosis matters before beginning treatment. Your podiatrist may take a nail sample and send it for pathology to confirm what’s actually going on.
What Causes a Fungal Toenail Infection?
Fungal toenail infections are caused by microscopic fungi that thrive in warm, dark, moist environments, which is exactly what the inside of a shoe provides. There are three main types responsible:
- Dermatophytes — the most common culprit, and the same group that causes tinea (athlete’s foot).
- Yeasts (such as Candida) — more common in fingernails, but can affect toenails.
- Non-dermatophyte moulds — less common, but require a different treatment approach.
Identifying the specific type matters because treatment differs depending on which fungus is present. That’s why your podiatrist may recommend pathology testing before finalising a treatment plan.
It Often Starts with a Skin Infection
In many cases, what causes a fungal toenail infection isn’t a nail problem at all, it’s a skin problem that spreads. Tinea pedis (athlete’s foot) lives in the skin between the toes and on the soles of the feet. Left untreated, this skin infection can migrate into the nail bed and establish itself in the nail.
Once the fungus is in both the skin and the nail, it creates a cycle of reinfection. Treating only the nail while ignoring the skin, or vice versa, is one of the main reasons fungal infections persist despite treatment.
Contamination from Shared Spaces and Equipment
Fungal spores are hardy and can survive on surfaces for extended periods. Common transmission points include:
- Public pools, gym change rooms, and communal showers.
- Nail salons that use unsterilised clippers or tools between clients.
- Sharing towels, socks, or footwear with someone who has an active infection.
Nail salons are a particularly underappreciated risk. If tools aren’t properly sterilised between customers, fungal spores can be introduced directly to the nail. If you get regular pedicures, it’s worth asking about sterilisation practices before your next appointment.
Who Is Most At Risk?
While anyone can develop one, understanding what causes a fungal toenail infection to take hold more easily comes down to a few key risk factors.
Age
Nails grow more slowly as we age, and the nail plate can become drier and more prone to small cracks that allow fungi to enter. People aged 60 and over are considerably more susceptible. Both because of slower nail turnover and longer cumulative exposure to fungal environments.
Diabetes and Circulation Problems
People with diabetes or poor circulation are at higher risk for two reasons: reduced blood flow means the immune system is slower to respond to infection at the extremities, and nerve damage can mean early signs go unnoticed. If you have diabetes, a fungal nail infection warrants prompt professional attention rather than a watch-and-wait approach.
A Weakened Immune System
Your immune system is your first line of defence against fungal colonisation. Illness, certain medications (such as immunosuppressants), or ongoing health conditions that compromise immune function can all lower your resistance and allow infection to establish more easily.
Footwear and Work Environment
Tight, poorly ventilated shoes create exactly the warm, damp conditions fungi love. Those who sweat heavily, wear occlusive footwear for long hours, or work in humid environments, such as kitchens or construction sites, face higher daily exposure. Choosing natural fibre socks and breathable footwear can make a genuine difference.
Why Toenails More Than Fingernails?
Toenails are far more commonly affected than fingernails. Shoes trap heat and moisture, toenails grow more slowly (giving infections longer to progress before the nail turns over), and toes receive significantly less direct sunlight, all conditions that favour fungal growth.
Minor repetitive trauma also plays a role. A nail repeatedly knocked against the front of a shoe — common in runners, hikers, or those who spend long periods on their feet — can develop small separations between the nail and nail bed, creating an easy entry point for fungal spores.
Can a Fungal Toenail Infection Spread?
Yes, and in more than one direction. Infections can spread from one toenail to adjacent nails, from the nail to the surrounding skin, and from person to person through shared surfaces or items.
This is why footwear hygiene matters even after treatment begins. Fungal spores can live inside shoes, reinfecting a healing nail if they aren’t properly sanitised. At Sole Podiatry, we offer UV shoe sanitising in clinic and encourage patients to bring their shoes in as part of the treatment process.
If your nail has already taken visible damage from the infection, KeryFlex may be the best treatment for fungal toenail restoration while your nail heals. Read more about it here.
When Should You See a Podiatrist?
Over-the-counter antifungal products can manage mild infections, but they have real limitations. Particularly for nails that are thickened, significantly discoloured, or where the infection has been present for some time. Consider booking an appointment if:
- The nail is thickening, crumbling, or lifting from the nail bed.
- Discolouration is spreading or involves multiple nails.
- You have diabetes, poor circulation, or a compromised immune system.
- You’ve tried topical treatments without success.
- The area around the nail is becoming painful or tender.
A podiatrist can confirm the diagnosis, identify the specific fungal type, and build a treatment plan that addresses both the nail and any underlying skin infection, giving you a much better chance of clearing it for good.
Not sure what treatment involves? Read our full guide to fungal toenail treatment to see what a professional treatment plan looks like.
Frequently Asked Questions
What causes a fungal toenail infection in the first place?
Fungal toenail infections are typically picked up through direct contact with fungal spores, in communal areas like public pools, gym change rooms, or nail salons using unsterilised tools. You can also develop one if you already have a skin fungal infection like athlete’s foot that spreads to the nail.
What does a fungal toenail infection look like?
The most common signs are a nail that has turned yellow, brown, or white, along with thickening, roughness, brittle edges, or lifting away from the nail bed. Keep in mind that other conditions like nail trauma or psoriasis can look similar, so it’s worth getting a proper diagnosis.
Can you treat a fungal toenail infection at home?
Mild cases can be managed with over-the-counter topical antifungals, but home treatment alone has significant limitations. Especially if the nail is already thickened or the infection has been present for a while. For best results, home treatments work most effectively as part of a broader plan guided by a podiatrist.
How do you get rid of a fungal toenail infection?
There’s no single fix. The most effective approach combines professional treatment (such as laser therapy and nail debridement) with topical antifungals and good foot hygiene, including treating any skin infection and sanitising your footwear. This multi-level approach is what gives you the best chance of clearing it for good.
Know the Cause. Stop the Cycle.
Fungal toenail infections don’t appear out of nowhere. They develop from specific fungi, often begin in the skin before moving into the nail, and are made more likely by a combination of environmental conditions, footwear habits, and individual health factors.
Once you understand what causes a fungal toenail infection, you’re better placed to address it, and to avoid the conditions that let it thrive in the first place. If you’re concerned about a nail that doesn’t look right, our team at Sole Podiatry can assess it, confirm what’s going on, and map out the most effective path forward.
