Nail fungus starts when microscopic fungi find a way past the hard outer surface of your nail and settle into the warm, protected nail bed underneath. About 10% of people worldwide deal with this infection at some point, and it accounts for roughly half of all nail disorders. The fungi responsible are nearly everywhere, from gym floors to your own shoes, but specific conditions have to line up for an infection to take hold.
How Fungi Actually Get Into Your Nail
Your nails have a natural seal where the nail meets the skin at the fingertip or toe tip. Over time, the dark, warm, moist environment inside shoes and repeated low-grade pressure on the nail weaken and break this seal, creating an entry point. The fungi produce enzymes that break down keratin, the tough protein your nails are made of. They typically start between the toes as athlete’s foot, spread across the sole, and then gradually reach that compromised seal at the nail’s edge.
Once through, the fungus works its way backward along the nail bed toward the cuticle. As it spreads, the nail begins to lift away from the bed and thickens underneath with a chalky buildup of debris. This is why infections usually start at the tip or side of the nail and progress inward over weeks or months rather than appearing all at once.
Small, everyday injuries to the nail speed this process up considerably. Stubbing a toe, wearing shoes that press against the nail, running or hiking in tight footwear: all of these create micro-trauma that cracks the nail’s defenses. You don’t need a dramatic injury. Even the repetitive impact of walking in poorly fitting shoes is enough.
Where You Pick It Up
The fungi that cause nail infections thrive on wet surfaces where people walk barefoot. Researchers have isolated them from pool decks, locker room floors, changing rooms, and even foot-washing stations at swimming facilities, where they persist despite regular chlorine disinfection. The highest concentrations show up along high-traffic walkways, especially near entrances and exits where foot traffic converges.
Your own home can be a source too. Studies have found that simply walking on carpets or bathroom floors after an infected household member is enough to transfer fungal spores. Sharing slippers is another common route. The fungi can even survive in washed socks and contaminated nail polish containers, meaning laundering doesn’t always eliminate the risk.
Athlete’s foot and nail fungus are closely linked. Having a fungal skin infection on your foot is one of the strongest predictors of developing a nail infection, because the fungus spreads directly from the surrounding skin into the nail. The reverse is also true: an untreated nail infection acts as a reservoir that keeps reinfecting the skin of your foot.
The Fungi Behind the Infection
About 90% of toenail infections and at least 50% of fingernail infections are caused by a group of fungi called dermatophytes. These are organisms specifically adapted to feed on keratin. The two species responsible for the vast majority of cases are Trichophyton rubrum and Trichophyton mentagrophytes.
Yeast, particularly Candida albicans, causes a smaller but notable share of infections, especially in fingernails. People who frequently have their hands in water (dishwashers, bartenders, healthcare workers) are more prone to yeast-driven nail infections because constant moisture softens the nail and weakens the surrounding skin. Various environmental molds can also infect nails, though they’re less common and sometimes harder to treat.
Why Some People Are More Vulnerable
Age is the single biggest demographic risk factor. As you get older, nails grow more slowly, thicken, and become drier and more brittle, all of which makes them easier for fungi to penetrate. Blood circulation to the extremities also decreases with age, which means your immune system delivers fewer infection-fighting cells to the nail bed.
Diabetes raises risk through several mechanisms at once. High blood sugar damages small blood vessels and impairs the white blood cells that would normally fight off a fungal invasion. At a molecular level, persistently elevated blood sugar leads to the accumulation of compounds that may actually help fungi stick to nail proteins, giving them a better foothold. People with diabetes also frequently develop nerve damage in their feet, which means they may not notice early signs of nail trauma or infection until it’s well established.
Other conditions that increase susceptibility include peripheral vascular disease (reduced blood flow to the feet), obesity, and any condition that suppresses immune function. Even something as simple as having sweaty feet or wearing occlusive footwear all day creates the warm, humid microenvironment fungi need. Fungal growth is most aggressive at temperatures between 20 and 30°C (68 to 86°F) with high humidity, which is exactly the climate inside a closed shoe on a warm day.
Why Your Shoes Are Part of the Problem
Fungi need moisture, warmth, and darkness to grow. A closed shoe checks all three boxes. The inside of an athletic shoe or leather work boot can reach the ideal temperature range for fungal growth within minutes of putting it on, and sweat provides the moisture. Several hours of this environment is enough for spore germination and the beginning of tissue invasion.
Wearing the same pair of shoes day after day without letting them dry out compounds the problem. Alternating between at least two pairs gives each one time to air out. Socks matter too. Moisture-wicking materials pull sweat away from the skin and help control the humid conditions fungi depend on. Copper-infused sock fibers have shown particular promise: in one study of college football players with athlete’s foot, wearing copper-fiber socks during practices led to gradual symptom reduction over eight weeks, with several participants experiencing complete resolution. Lab testing has shown these fibers can kill 99.9% of the fungus that causes athlete’s foot within 12 hours.
Nail Fungus vs. Other Nail Problems
Not every thick, discolored, or crumbly nail is fungal. Nail psoriasis, physical trauma, and even aging can cause changes that look remarkably similar. The most common signs of a fungal infection are thickening under the nail, the nail pulling away from the bed, and color changes (white, yellow, or brown). Psoriasis tends to produce some distinct features like tiny pits or dents in the nail surface, small red spots near the base of the nail, and reddish-brown “salmon patches” visible through the nail plate.
The overlap is significant enough that dermatologists typically recommend laboratory testing, usually a nail clipping examined under a microscope or sent for culture, before starting treatment. This distinction matters because antifungal medications won’t help psoriasis, and psoriasis treatments won’t clear a fungal infection. If your nails have changed and you’re not sure why, getting the right diagnosis first saves months of ineffective treatment.
Practical Steps That Reduce Your Risk
- Wear sandals or shower shoes in locker rooms, pool areas, and shared showers. The highest fungal concentrations are on the walkways where foot traffic is heaviest.
- Keep feet dry. Change socks when they’re damp, and choose moisture-wicking or copper-infused fabrics over cotton, which holds moisture against the skin.
- Rotate your shoes. Giving each pair at least 24 hours to dry between wearings deprives fungi of the consistent moisture they need.
- Treat athlete’s foot early. A skin infection between your toes is often the direct source of a nail infection. Clearing it quickly removes the fungal reservoir.
- Wash your feet daily. In studies of people with diabetes, daily foot washing was associated with significantly lower rates of nail fungus, with one study finding it reduced odds by more than three-fold.
- Trim nails straight across and avoid cutting them too short. Ragged edges and overly short nails create micro-injuries that give fungi an entry point.
- Don’t share nail clippers, files, or slippers with household members, especially if anyone in the home has a known infection.