Toenail fungus starts when microscopic fungi, usually a type of mold called a dermatophyte, find their way into the nail through tiny gaps or damage in the surrounding skin. These organisms feed on keratin, the protein that makes up your nails, and once established they’re notoriously difficult to clear. About half of all people over 70 will deal with a fungal nail infection at some point, but it can happen at any age, especially if the conditions are right.
How Fungi Get Into the Nail
Dermatophytes cause about 90% of toenail fungal infections, though yeasts and other molds are sometimes responsible. The infection typically begins in the skin just under the tip of the nail, where the nail meets the nail bed. From there, fungal threads work their way into the underside of the nail plate, breaking down the material between cells and creating larger pores in the tissue. This weakens and separates the nail from below, which is why an infected nail often looks thickened, discolored, or crumbly.
The fungi don’t need a dramatic wound to get in. A minor nick from trimming your nails too short, a small crack from stubbing your toe, or chronic pressure from tight shoes can all create enough of an opening. Once the fungus reaches keratin-rich tissue, it has a steady food source and a protected environment that’s hard for your immune system to reach.
The Places Where You Pick It Up
Fungal spores thrive in warm, damp environments and can survive on surfaces for extended periods. The most common pickup spots are public showers, pool decks, locker rooms, and gym floors. When someone with a foot infection walks barefoot on a wet surface, they leave behind spores that stick around until the next bare foot comes along.
Nail salons are another well-documented source. Tools like nail clippers, files, and cuticle pushers can carry fungal material from one client to the next if they aren’t properly disinfected between appointments. Foot basins and spa jets can harbor organisms too. OSHA specifically recommends that salon workers clean and disinfect all tools after each client, but enforcement varies widely.
You can also pick up the fungus from shared shoes, borrowed socks, or even a contaminated bath mat at home. If someone in your household has a toenail infection or athlete’s foot, the shower floor becomes a potential transmission route for everyone.
Athlete’s Foot Is the Most Common Gateway
One of the most frequent paths to toenail fungus is an existing case of athlete’s foot. The two conditions are caused by the same organisms, and a history of athlete’s foot more than doubles your risk of developing a toenail infection. The fungus starts as itchy, scaly skin between your toes or on the soles of your feet, then gradually migrates to the nails if left untreated.
This self-spreading pattern is why dermatologists look for both conditions at the same time. Treating just the nail while ignoring a skin infection on the same foot often leads to reinfection, since the fungus simply recolonizes the nail from the surrounding skin.
Why Your Shoes Make It Worse
The inside of a closed shoe is essentially a perfect habitat for fungi: warm, dark, and damp. Your feet produce sweat throughout the day, and shoes with limited airflow trap that moisture against your skin for hours at a time. Add tight socks or synthetic materials that don’t wick moisture, and fungal growth accelerates.
People who spend long hours on their feet in work boots, athletic shoes, or steel-toed footwear are at higher risk simply because of the sustained environment. Rotating between pairs of shoes so each pair has time to dry out completely, and choosing moisture-wicking socks, reduces the conditions fungi need to establish themselves.
Risk Factors That Raise Your Odds
Age is the single most common risk factor. As you get older, your nails grow more slowly, your circulation to your feet decreases, and your immune system becomes less effective at fighting off fungal invaders. All of these changes give the fungus more time and opportunity to take hold.
Several medical conditions also increase susceptibility. Diabetes affects blood flow to the feet and can reduce immune function in the skin. People with weakened immune systems are especially vulnerable. One in four people living with HIV develops a fungal nail infection. Conditions that affect leg circulation, including venous insufficiency, also raise the risk.
Other factors that make infection more likely include:
- Repeated nail trauma from running, hiking, or ill-fitting shoes that press on the toenails
- Obesity, which has been linked to significantly higher rates of nail fungus in clinical studies
- Genetic predisposition, meaning some families are simply more susceptible to these infections
- Occupational exposure, particularly jobs that involve wet environments or heavy, enclosed footwear
Why It Keeps Coming Back
Even after successful treatment, toenail fungus returns in roughly 10% to 53% of cases, depending on the study. The most reliable estimates put the recurrence rate at 20% to 25%. Reinfection typically happens within about three years of clearing the original infection.
Recurrence happens for two main reasons. First, the same environmental exposures that caused the initial infection are usually still present: the same shoes, the same gym shower, the same household bathroom. Second, the risk factors that made you vulnerable in the first place, like age, circulation issues, or slow nail growth, don’t change after treatment. Without addressing both the fungus itself and the conditions that invited it in, the cycle tends to repeat.
People who still have athlete’s foot when their nail infection clears are particularly likely to see the fungus return, since the skin around the nail remains a reservoir of the same organisms.
Practical Ways to Lower Your Risk
Wear flip-flops or shower shoes in public showers, pool areas, and locker rooms. Dry your feet thoroughly after bathing, including between the toes, since residual moisture is one of the easiest risk factors to eliminate. If you notice itchy, peeling skin between your toes, treat it promptly with an over-the-counter antifungal before it has a chance to spread to the nails.
Keep your toenails trimmed short and straight across. Avoid cutting the cuticles or digging into the corners, which creates micro-injuries the fungus can exploit. If you get pedicures, choose salons that visibly sterilize tools between clients or bring your own set.
Swap out old athletic shoes that have absorbed months of sweat. If your feet sweat heavily, changing socks midday and using absorbent foot powder can keep moisture levels low enough to discourage fungal growth. For people who’ve already had one infection, these habits aren’t optional extras. They’re the difference between a one-time problem and a recurring one.