Athlete’s foot is caused by a group of fungi called dermatophytes that feed on keratin, the tough protein in your outer layer of skin. One species in particular is responsible for roughly 70% of cases. The infection thrives in warm, moist conditions, which is why it targets feet trapped inside shoes all day and spreads so easily in locker rooms and pool areas.
The Fungi Behind the Infection
Several related fungal species cause athlete’s foot, but one dominates. A fungus called Trichophyton rubrum accounts for about 70% of all cases. The remaining infections come mostly from two other species, T. interdigitale and Epidermophyton floccosum. These organisms are specialized skin parasites. They produce enzymes that break down keratin, allowing them to burrow into the outermost layer of skin and use the broken-down protein fragments as fuel.
A newer species, T. indotineae, has recently appeared in the United States after being identified in India, Canada, and the Middle East. It’s worth noting because it resists standard antifungal treatments, making infections harder to clear.
How the Fungus Gets Onto Your Feet
Athlete’s foot spreads through both direct and indirect contact. Direct transmission happens when your bare skin touches someone else’s infected skin or a flake of shed skin carrying the fungus. Indirect transmission happens through contaminated objects: shared shoes, socks, towels, or bedding.
Public environments are the most common source. Pool decks, locker room floors, communal showers, and saunas are high-risk surfaces because many people walk barefoot and moisture keeps the fungus alive. These fungi are remarkably resilient. In lab studies, T. rubrum and T. mentagrophytes survived at least 123 days in chlorinated swimming pool water kept at normal pool temperatures (28 to 30°C). Standard chlorine and ozone concentrations used to control bacteria did not kill them. That means the fungus can persist on wet surfaces for months, waiting for bare feet.
Why Moisture and Heat Matter So Much
The inside of your shoe creates exactly the environment these fungi need. Research measuring conditions inside different types of footwear found that closed shoes had significantly higher temperatures and humidity than open footwear like sandals. Cloth shoes were the worst offenders, reaching an average internal temperature of 32.2°C and humidity of 82.3%. People whose shoes hit 32°C or higher with 80% humidity or above had significantly higher rates of infection.
Lab studies confirm why. Dermatophytes need humidity of at least 90% to penetrate intact skin. Below 85%, they can’t infiltrate at all, and at 80%, invasion slows dramatically. But here’s the catch: if you have any small cut, crack, or abrasion on your foot, the fungi can penetrate skin at just 70% humidity. That’s easily reached inside most closed shoes on a warm day, which is why minor skin damage on the feet is such a significant risk factor. Once the fungi get past the surface, their invasion rate increases proportionally with rising humidity.
Who Gets Athlete’s Foot and Why
The condition is most common among athletes (especially those who walk barefoot around pools and locker rooms), military personnel, manual laborers, people in long-term care facilities, and homeless individuals. The common thread is prolonged time in closed footwear, shared facilities, or limited access to foot hygiene.
Excessive sweating on the feet is one of the strongest individual risk factors. A study comparing people with and without athlete’s foot found that those with the infection were 3.5 times more likely to have plantar hyperhidrosis, a condition of excessive foot sweating. Over half of athlete’s foot patients in the study had measurable hyperhidrosis, compared to a third of people without the infection. If you notice your feet are consistently damp even in moderate conditions, that alone raises your risk considerably.
Several other factors make infection more likely:
- Prolonged water exposure on the feet, which softens and weakens the skin barrier
- Skin conditions like eczema or psoriasis, which compromise the skin’s protective layer
- Diabetes or poor circulation, which reduce the skin’s ability to fight off infection
- Weakened immune function, whether from illness or medication
- Obesity, which increases moisture in skin folds and may limit foot care
- Genetic susceptibility, meaning some people are inherently more prone to fungal skin infections
How Infection Develops After Exposure
After fungal spores land on your foot, they don’t cause symptoms right away. The fungi first need to attach to the skin surface, then use their keratin-digesting enzymes to break through the outer layer. In controlled lab conditions, both major species can enter the outer skin within a single day when humidity is high enough. In real life, it typically takes longer because conditions inside your shoe fluctuate and your immune system puts up some resistance.
Most people first notice itching and scaling between the toes, particularly between the fourth and fifth toes where moisture collects most easily. This is the most common pattern. A second form shows up as small, fluid-filled blisters on the soles, often caused by T. mentagrophytes. A third pattern produces a dry, scaly rash across the bottom of the foot that many people mistake for simple dry skin.
Once established, the fungus can persist indefinitely if untreated. T. rubrum in particular tends to cause chronic, low-grade infections that flare and subside over months or years. The fungus can also spread from the feet to the toenails, groin, or hands, especially if you touch your infected feet and then touch other areas of your body.
Reducing Your Exposure
Since the fungus survives for months on surfaces and standard pool chemicals don’t kill it, prevention comes down to limiting contact and controlling the environment on your feet. Wearing sandals or flip-flops in communal showers, locker rooms, and pool areas is the single most effective step. Keeping feet dry matters almost as much. Changing socks when they become damp, choosing moisture-wicking materials, and alternating shoes so each pair has time to dry out between wears all reduce the interior humidity that fungi depend on.
Open footwear like sandals and slippers consistently showed the lowest internal temperature and humidity in studies, so wearing them when practical gives fungi the least hospitable environment. If you’re prone to sweaty feet, addressing the moisture problem directly (with absorbent foot powders or moisture-wicking socks) can meaningfully lower your risk, given the strong link between excessive sweating and infection. Avoiding shared towels, shoes, and socks eliminates another common route of transmission.