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, Trichophyton rubrum, is responsible for roughly 70% of cases. Trichophyton interdigitale causes most of the rest, with a handful of less common species making up a small percentage.
How the Fungus Attacks Your Skin
Dermatophytes survive by breaking down keratin, and they’re very good at it. These fungi produce specialized enzymes called keratinases that snip apart the protein bonds holding your outer skin together, essentially dissolving the surface layer to use as food. This is why athlete’s foot stays shallow: the fungus has no interest in deeper tissue, only the dead, keratin-rich cells on the surface. As the fungus digests its way through, the skin responds with inflammation, which is what produces the itching, redness, peeling, and flaking you actually feel.
Your immune system does fight back. Skin cells detect the invading fungus and trigger mast cells to release histamine and other inflammatory signals, recruiting white blood cells to the site. In most healthy people, this immune response keeps the infection contained to a small area. But it also explains the persistent itch: histamine release is the same process behind allergic reactions, and the fungus keeps triggering it as long as it’s present.
Why Feet Are the Perfect Target
Dermatophytes need warmth and moisture to penetrate skin, and the inside of a shoe delivers both. A study published in The Journal of Dermatology measured conditions inside footwear and found that people with athlete’s foot had significantly higher shoe temperatures (averaging about 32°C) and humidity levels (averaging 76%) compared to people without the infection. When humidity inside footwear topped 75%, the odds of having athlete’s foot roughly tripled.
Lab experiments confirm this relationship. At 80% humidity, the fungi penetrate skin slowly. At 90% humidity or above, they infiltrate and multiply readily. This is why the spaces between your toes are the most common site of infection: those narrow gaps trap moisture and warmth, regularly reaching 80% humidity or higher, especially in summer. Notably, if the skin is already damaged by a crack or blister, the fungus can establish itself even at lower humidity levels, around 70%.
Where You Pick It Up
The fungus spreads through direct contact with contaminated surfaces or skin. Locker rooms, pool decks, shared showers, and gym floors are classic transmission sites because people walk barefoot on warm, wet surfaces where infected skin cells have been shed. But transmission isn’t limited to public places. Sharing towels, socks, or shoes with someone who has athlete’s foot can pass the fungus along just as easily.
What makes dermatophytes especially persistent is their survival outside the body. Fungal spores can withstand heat drying in standard household dryers, freezing at -20°C for a week, and even direct heat exposure at 60°C for 90 minutes without dying. One study found dermatophyte spores remained viable after being frozen for over 20 years. Washing contaminated fabrics at 40°C (a typical warm cycle) does not kill them. Only laundering at 60°C or higher for extended cycles reliably eliminates the fungi. This means your socks, towels, and bed linens can harbor the fungus far longer than you might expect.
Risk Factors That Make Infection More Likely
Anyone can get athlete’s foot, but certain factors raise your chances considerably. Wearing occlusive footwear (closed-toe shoes, boots, or synthetic materials that don’t breathe) for long hours creates the high-humidity microenvironment dermatophytes need. Athletes and military personnel have historically high rates for exactly this reason.
Diabetes is a major risk factor. Among diabetic patients in Saudi Arabia, the prevalence of athlete’s foot was 15.5%, and an Egyptian study found it reached 42.6% in diabetics. Compare that to the estimated global prevalence of about 3% in the general population. Diabetes impairs blood flow to the feet and can reduce the immune response in the skin, giving the fungus an easier foothold. Poor circulation from other causes has a similar effect.
A weakened immune system from any cause, whether from medication, illness, or aging, also increases susceptibility. People who have already had athlete’s foot are prone to reinfection, partly because the fungus can persist in toenails (a condition called onychomycosis) and recolonize the skin repeatedly.
Different Forms, Different Causes
Athlete’s foot doesn’t always look the same, and the specific fungal species involved can influence how the infection presents.
- Interdigital type: The most common form. It appears as peeling, maceration (soggy white skin), and fissures between the toes, especially in the outer toe clefts. T. rubrum is usually the culprit.
- Moccasin type: A chronic form with dry, scaly skin across the sole, heel, and sides of the foot, sometimes resembling very dry skin rather than an obvious infection. T. rubrum is again the primary cause, and this type tends to be stubborn and long-lasting.
- Vesiculobullous type: Characterized by painful, fluid-filled blisters on the sole. This form is more often caused by T. interdigitale and tends to flare up suddenly rather than building gradually.
If the skin between the toes cracks deeply enough, bacteria can enter and cause a secondary infection on top of the fungal one. This can lead to an acute ulcerative form with more severe pain, swelling, and drainage, a complication that requires treatment for both the fungus and the bacteria.
Why It Keeps Coming Back
Athlete’s foot has a reputation for recurring, and the biology explains why. The fungal spores are extraordinarily durable on surfaces and in fabrics. If you treat the skin but continue wearing contaminated shoes or reusing towels washed at low temperatures, you’re reintroducing spores to freshly healed skin. The fungus can also quietly persist in thickened toenails, acting as a reservoir that reseeds the surrounding skin after treatment ends.
The environment inside your shoes doesn’t change just because the infection clears. If your feet still spend hours in warm, damp conditions, the same factors that caused the original infection remain in place. Preventing recurrence means addressing the cause at every level: reducing moisture, decontaminating fabrics and footwear, and treating any toenail involvement that could serve as a source of reinfection.