Toenail fungus starts when microscopic fungi, usually a group called dermatophytes, get under your nail through tiny cracks or separations between the nail and the nail bed. These organisms thrive in warm, damp, dark environments, which is exactly the climate inside most shoes. About 5.5% of people worldwide have a toenail fungal infection at any given time, though rates are much higher in certain groups.
How Fungi Get Into Your Nail
The infection almost always begins at the tip or edge of the toenail, appearing as a white or yellow-brown spot. Fungi don’t bore through healthy, intact nail on their own. They need an entry point: a small gap between the nail and the skin beneath it, a crack from trimming too aggressively, or damage from stubbing your toe or wearing shoes that press against the nail repeatedly.
Once inside, the fungus feeds on keratin, the protein that makes up your nail. It gradually works deeper into the nail bed, causing the nail to thicken, discolor, and become brittle or crumbly. The process is slow. Most people don’t notice anything until the infection has been established for weeks or months. Dermatophytes in the genus Trichophyton cause 80% to 90% of all cases, with the remainder caused by yeasts and molds.
Where You Pick It Up
Fungal spores spread easily in warm, wet communal areas. Pool decks, gym showers, locker room floors, and shared bathing spaces in dorms are common transmission sites. When someone with a fungal infection walks barefoot on these surfaces, they leave spores behind. If you walk the same floor with bare feet, the spores can land on your skin and eventually migrate to your nails.
Sharing nail clippers, towels, or shoes with someone who has a fungal infection is another route. Many people also develop toenail fungus as an extension of athlete’s foot. The same fungi cause both conditions, and an untreated skin infection between the toes can spread to the nail over time. Treating athlete’s foot promptly is one of the most practical ways to prevent toenail fungus from developing.
The Shoe Problem
Your footwear plays a bigger role than most people realize. Fungi need darkness, warmth, and moisture to grow, and a closed shoe delivers all three. Heavy leather work boots and steel-toed safety shoes are especially problematic because they don’t breathe. Pair those with thick cotton socks that absorb sweat and hold it against your skin, and you’re essentially incubating fungal spores for eight to ten hours a day.
That persistent moisture does more than just feed the fungus. It softens the nail plate, making it easier for organisms to invade deeper into the nail bed. Winter can be surprisingly risky because insulated boots trap even more heat and moisture than summer footwear. People who notice a nail changing color in late winter or early spring often picked up the infection months earlier, when their feet were sealed inside heavy boots for long stretches.
Health Conditions That Raise Your Risk
Diabetes is one of the strongest risk factors. People with diabetes are roughly three times as likely to develop toenail fungus compared to those without it. In one study, 22% of elderly diabetic patients had the infection. Diabetes reduces blood flow to the feet and can dampen the immune response in the extremities, giving fungi a foothold that a healthy immune system might otherwise fight off.
Poor circulation from peripheral arterial disease (PAD) carries a similar risk. In patients visiting a vascular clinic for PAD, 35.1% had toenail fungus. Even after accounting for smoking, which itself damages circulation, PAD remained a strong independent predictor of infection. Heavy smokers face compounded risk: in one study, 83.3% of people smoking at least two packs a day had toenail fungus.
Anyone taking medications that suppress the immune system, including long-term corticosteroids or drugs used after organ transplants, is more vulnerable. The immune system normally keeps fungal organisms in check on the skin’s surface, and when that defense weakens, infections take hold more easily and spread faster.
Age and Genetics Matter Too
Toenail fungus becomes increasingly common as you get older. Nails grow more slowly with age, giving fungi more time to establish themselves. Older adults also tend to have reduced circulation, thicker nails that are more prone to cracking, and conditions like arthritis or poor vision that make routine foot care harder. All of these factors compound over decades.
There’s also a genetic component that researchers are still working to understand. People are not equally susceptible to fungal infections even when they share the same risk factors and environments. Studies have found evidence of familial predisposition to dermatophyte infections, mediated by differences in how the immune system recognizes and responds to fungi. If toenail fungus runs in your family, you may be inherently more susceptible regardless of your habits.
How to Lower Your Risk
Prevention comes down to reducing moisture and limiting exposure to fungal spores. Wear sandals or shower shoes in gym showers, pool areas, and hotel bathrooms. Dry your feet thoroughly after bathing, especially between the toes. Keep toenails trimmed short and straight across so there are fewer gaps for fungi to enter.
Choose socks made from moisture-wicking or absorbent materials and change them if they get damp during the day. If your work requires heavy boots, consider rotating between two pairs so each has time to dry out completely. Over-the-counter antifungal creams and powders marketed for athlete’s foot can’t treat an established nail infection because they can’t penetrate the nail plate, but they can be useful as a preventive measure on the surrounding skin.
If you already have athlete’s foot, treating it early is one of the most effective things you can do. The same fungi responsible for that itchy, flaking skin between your toes will happily migrate to a nearby nail if given enough time.