Toenail fungus is caused by dermatophytes in 80% to 90% of cases, with one species dominating above all others: Trichophyton rubrum. These fungi have evolved specifically to feed on keratin, the tough protein that makes up your nails, skin, and hair. The remaining cases are caused by yeasts or environmental molds, each producing slightly different patterns of infection.
Dermatophytes: The Primary Culprits
Dermatophytes are a specialized group of fungi that can only survive on keratinized tissue. Unlike mold on bread or yeast in beer, these organisms have no interest in sugars or starches. They produce enzymes that break down keratin into nutrients they can absorb, which is why they target nails, skin, and hair exclusively. That enzymatic digestion of your nail’s protein structure is what causes the characteristic thickening, yellowing, and crumbling.
Three genera of dermatophytes cause nail infections: Trichophyton, Epidermophyton, and Microsporum. In practice, though, the picture is simpler than that list suggests. Trichophyton rubrum is responsible for the vast majority of toenail infections worldwide. It’s classified as “anthropophilic,” meaning it has adapted specifically to human hosts and spreads person to person rather than from animals or soil.
Two other species round out the most common dermatophytes in toenails:
- Trichophyton mentagrophytes causes a distinct pattern called white superficial onychomycosis, where chalky white patches form on the nail surface rather than underneath it. This species can also spread from animals to humans.
- Epidermophyton floccosum is another human-adapted species that occasionally infects nails, though it’s more commonly associated with skin infections like jock itch.
If you’ve ever had athlete’s foot, the connection to toenail fungus is direct. Athlete’s foot is most often caused by the same organism, T. rubrum. The fungus can migrate from the skin between your toes into the nail bed, where it’s far harder to eliminate.
Yeasts and Non-Dermatophyte Molds
The 10% to 20% of cases not caused by dermatophytes fall into two categories. Candida albicans, a yeast that normally lives on your skin and in your gut, can infect nails. Candida nail infections are more common on fingernails than toenails and tend to affect people whose hands are frequently wet.
Non-dermatophyte molds like Scopulariopsis and Aspergillus species can also colonize damaged nails. These are environmental fungi found in soil and decaying matter. They typically don’t infect healthy nails on their own but can take hold when a nail is already compromised by injury, a pre-existing dermatophyte infection, or poor circulation. Identifying which type of fungus you have matters because some molds don’t respond to the same medications that work against dermatophytes.
How the Infection Takes Hold
The most common pattern is distal subungual onychomycosis, where T. rubrum invades the nail bed starting at the tip of the toe and works its way back toward the cuticle. The fungus grows in the layer of skin beneath the nail plate, triggering your body to produce extra keratin in response. That overproduction is what causes the nail to thicken and lift away from the nail bed. Debris builds up underneath, giving the nail its yellow or brownish discoloration.
A less common but distinctive pattern, proximal subungual onychomycosis, starts near the cuticle and grows outward. This form is also usually caused by T. rubrum and can sometimes signal an underlying immune deficiency. White superficial onychomycosis, caused by T. mentagrophytes, stays on the nail’s outer surface and is generally the easiest form to treat because the fungus hasn’t penetrated deep into the nail bed.
Where You Pick It Up
Dermatophytes thrive in warm, damp environments. Public showers, pool decks, locker room floors, and shared bathing areas are classic transmission sites. The fungi shed in tiny skin flakes from infected people and can survive on wet surfaces long enough for someone else to pick them up barefoot. Once on your skin, they need the right conditions to establish an infection: warmth, moisture, and darkness. Closed-toe shoes and sweaty socks create exactly that environment.
Changing wet socks after exercise, wearing sandals in communal showers, and keeping your feet dry all reduce your exposure. But even with precautions, toenail fungus is remarkably common. It affects 2% to 5% of the population at any given time, with a lifetime prevalence as high as 20%. The risk climbs with age, peaking in people over 60. Slower nail growth, reduced blood flow to the feet, and decades of cumulative exposure all contribute to higher rates in older adults.
How Doctors Identify the Fungus
Not every thick or discolored toenail is fungal. Psoriasis, trauma, and other conditions can mimic the appearance. Confirming the diagnosis, and identifying which fungus is involved, typically starts with a nail clipping. The most common initial test dissolves the nail sample in potassium hydroxide (KOH) to reveal fungal structures under a microscope, with detection rates around 87.5%. This confirms the presence of fungus but doesn’t always identify the exact species.
Fungal culture, where the sample is grown in a lab, can pinpoint the species but is notoriously slow (taking weeks) and has a low success rate of under 10% in some studies. Newer DNA-based testing (PCR) offers faster species identification with detection rates comparable to microscopy, around 84%. Knowing whether you’re dealing with a dermatophyte, a yeast, or a mold helps guide which treatment has the best chance of working.
Why the Fungus Type Affects Treatment
Oral antifungal medications are the most effective option for toenail infections, but cure rates vary widely. For dermatophyte infections, oral terbinafine taken daily for 12 weeks produces clinical cure rates of 38% to 76% for toenails. Itraconazole, another oral option, achieves 14% to 63%. Those ranges reflect how stubborn toenail fungus is. Even the best available medications fail a significant portion of the time, partly because toenails grow slowly (6 to 18 months for full replacement) and the fungus can persist in the nail bed.
Topical treatments applied directly to the nail are less effective. The nail plate acts as a physical barrier that topical medications struggle to penetrate. Topical options used daily for 48 weeks produce cure rates in the single digits to low teens. They’re most useful for mild or superficial infections, particularly white superficial onychomycosis where the fungus sits on the nail surface.
Non-dermatophyte molds present a particular challenge. Standard antifungals are designed to target dermatophyte biology, and some molds are naturally resistant. If a first course of treatment fails, species identification through culture or PCR becomes especially important to rule out a mold infection that may need a different approach.