Toenail fungus is caused by microscopic fungi that feed on keratin, the tough protein your nails are made of. A group of fungi called dermatophytes are responsible for about 90% of toenail infections, with one species in particular accounting for the vast majority of cases. The infection affects roughly 10% of the global population, and that number climbs sharply with age: more than 48% of people over 70 have it.
The Fungi Behind the Infection
The primary culprit is a dermatophyte called Trichophyton rubrum. This organism has evolved specifically to live on human skin and nails, and it’s the single most common cause of toenail fungus worldwide. A related species, Trichophyton mentagrophytes, is the second most frequent cause. Together, these two fungi account for the overwhelming majority of cases.
Yeasts and molds play a smaller role. Yeast infections (mainly Candida) cause only about 2% of nail infections and tend to affect fingernails rather than toenails. Nondermatophytic molds, including species of Fusarium and Aspergillus, account for roughly 8% of nail infections and are cultured primarily from toenails. These mold infections can be harder to treat because they don’t always respond to standard antifungal medications designed for dermatophytes.
How Fungus Breaks Down Your Nail
Your toenails are built from tightly packed keratin filaments held together by sulfur bridges, which is what makes them hard and resilient. Dermatophytes have developed a sophisticated chemical toolkit to dismantle this structure. The process starts with enzymes that break the sulfur bonds holding keratin filaments together, essentially loosening the nail’s internal scaffolding. Once those bonds are broken, a team of at least three different protein-digesting enzymes works together to chop the exposed keratin into smaller and smaller pieces that the fungus can absorb as food.
Additional enzymes attack the sugar-based connections between keratin building blocks, causing the filaments to fall apart entirely. The fungus also produces sulfite, a chemical that further weakens the remaining sulfur bridges and speeds up the whole process. This is why an infected nail gradually becomes thick, crumbly, and discolored: the fungus is literally eating it from the inside.
How Fungus Gets Into the Nail
Not all toenail infections start in the same place, and understanding the entry point helps explain why the infection looks different from person to person.
The most common route is through the tip of the nail. The fungus enters where the nail meets the skin at the fingertip, then slowly works its way backward toward the base. This produces the classic pattern of yellowing and thickening that starts at the free edge and creeps inward over months. About 10% of cases involve a different pattern where the fungus attacks the surface of the nail plate directly, producing chalky white patches on top of the nail.
A less common but more concerning entry point is through the cuticle at the base of the nail. The fungus penetrates the newly forming nail and spreads outward. This type is sometimes associated with weakened immune function and warrants closer medical attention. Yeast infections take yet another path: they attack the soft tissue surrounding the nail first, then invade the nail plate secondarily.
Where You Pick It Up
Dermatophyte spores are remarkably persistent. Some species can survive in shed skin scales for years under the right conditions. Researchers have detected T. rubrum in 86% of house dust samples from patients with athlete’s foot, and T. mentagrophytes in 81%. Both species have been cultured from the bare feet of people who simply walked through public baths, and ordinary washing and drying did not fully eliminate the spores.
Mold spores are somewhat less durable on surfaces, surviving anywhere from 2 to over 30 days depending on the material. But they’re ubiquitous in soil and outdoor environments, which is why gardeners and people who walk barefoot outside are also at risk. The common thread across all these organisms is that warm, moist environments favor transmission, and the fungus needs only a tiny break in the skin or nail to establish itself.
Why Your Shoes Matter
The environment inside your footwear plays a direct role in whether a fungal exposure turns into an infection. Research on dermatophyte growth rates shows that T. rubrum and T. mentagrophytes can penetrate the outer skin layer within a single day at 90% humidity and body temperature. At 85% humidity or below, infiltration drops to zero.
That 5% humidity gap is the difference between your shoes being a petri dish or a barrier. Occlusive footwear (think rubber boots, synthetic athletic shoes, or any shoe worn all day without ventilation) easily pushes internal humidity past that 90% threshold. This is a major reason toenails are infected far more often than fingernails: your feet spend hours each day in exactly the conditions fungi need to thrive.
Medical Conditions That Raise Your Risk
Diabetes is one of the strongest medical risk factors for toenail fungus, and the connection goes deeper than most people realize. High blood sugar causes two problems at once. First, it damages small blood vessels, reducing circulation to the feet and limiting the immune cells that can reach the area. Second, it impairs the white blood cells that would normally detect and kill fungal invaders.
At a molecular level, persistently elevated blood sugar leads to a buildup of sugar-modified proteins in the nail itself. These modified proteins may actually help fungal spores stick to the nail by providing extra binding sites. They also appear to interfere with the immune system’s ability to recognize the fungus as a threat. Studies have confirmed that diabetic patients have significantly higher levels of these modified proteins in their nails compared to non-diabetic individuals.
Peripheral artery disease, which reduces blood flow to the extremities for reasons beyond diabetes, carries a similar risk. So does any condition or medication that suppresses the immune system, including organ transplant drugs and HIV.
Age, Genetics, and Personal Susceptibility
Age is the single strongest predictor of toenail fungus. About 32% of people between 60 and 70 are affected, rising to more than 48% after age 70. Slower nail growth, reduced blood circulation, and decades of cumulative exposure all contribute. Older nails also tend to be thicker and more brittle, creating microscopic cracks that serve as entry points.
Genetics play a real but less visible role. Studies have identified an autosomal dominant pattern of inheritance linked to T. rubrum infections, meaning if one of your parents had chronic toenail fungus, your risk is elevated. Research on families with high susceptibility found a specific mutation in a gene called Dectin-1, which encodes a receptor the immune system uses to recognize fungal cells. People with this mutation produce less of the immune signaling molecules needed to mount an effective response. In a study comparing 43 people with toenail fungus to 30 healthy controls, those with the infection had double the circulating levels of regulatory T cells, a type of immune cell that dials down the body’s defensive response.
Toenail Fungus vs. Nail Psoriasis
Toenail fungus and nail psoriasis can look similar enough that even experienced clinicians sometimes need lab testing to tell them apart. Both can cause discoloration, thickening, and separation of the nail from the nail bed. But there are practical differences worth knowing.
- Fungal nails tend to be thick, crumbly or cracked, yellow to brown in color, and sometimes foul-smelling. The infection typically starts at one nail and may spread to others over time.
- Psoriatic nails are more likely to show pitting (small dents in the surface), horizontal grooves, or reddish-brown spots under the nail called oil drop spots. They tend to appear on fingernails rather than toenails, and most people with nail psoriasis also have psoriasis skin plaques elsewhere on the body or joint pain from psoriatic arthritis.
A definitive diagnosis usually requires clipping a small piece of nail or scraping material from underneath it for microscopic examination. If results are unclear, a small tissue biopsy from the nail bed can confirm whether fungus is present. This distinction matters because the treatments for fungal infections and psoriasis are entirely different, and using the wrong one wastes months while the condition progresses.