Nail fungus is caused by microscopic fungi that invade the nail through tiny cracks or separations between the nail and the skin beneath it. About 10% of adults worldwide have a fungal nail infection at any given time, making it the single most common nail disease. The same group of fungi responsible for athlete’s foot causes the vast majority of cases, and the infection typically starts at the tip of the nail before slowly spreading toward the base.
The Fungi Behind the Infection
A family of fungi called dermatophytes accounts for roughly 90% of toenail infections and about 50% of fingernail infections. These organisms evolved from soil-dwelling molds that fed on dead protein debris in the ground. Over time, they developed the ability to break down keratin, the tough structural protein that makes up your nails, skin, and hair. One species in particular dominates: it produces enzymes that dissolve keratin, giving the fungus a way to penetrate the nail plate and feed on the tissue beneath.
Yeasts, most commonly the same type responsible for oral thrush, cause a small percentage of nail infections, particularly in fingernails. Another group of environmental molds found in soil and decaying plant matter can also infect nails, accounting for about 8% of cases. These molds were long dismissed as harmless contaminants when they showed up in lab cultures, but researchers now recognize them as genuine nail pathogens, especially in warmer climates.
How Fungus Gets Into the Nail
The most common entry point is the far edge of your nail, where the nail plate meets the skin at the tip of your finger or toe. Fungi slip into the nail bed from underneath, then slowly migrate toward the cuticle. This is why most fungal nails first show discoloration or thickening at the tip before the changes creep backward over weeks or months.
Less commonly, fungi can enter through the cuticle area at the base of the nail and work their way forward. This pattern is more typical in people with weakened immune systems and can sometimes be an early sign of immune suppression. A third route involves fungi colonizing the surface of the nail plate directly, producing chalky white patches on the top of the nail rather than the yellowed, thickened appearance of deeper infections.
Any break in the nail’s natural barrier creates an opening. A stubbed toe, a nail trimmed too aggressively, repeated pressure from tight shoes, or even a minor separation of the nail from the nail bed can be enough. Once inside, the fungi produce protein-dissolving enzymes that break apart the keratin structure, creating space to grow deeper while generating the crumbly, discolored debris that builds up under infected nails.
Where You Pick It Up
Fungi that infect nails thrive in warm, damp environments. Communal showers, pool decks, locker room floors, and shared bathing areas are classic transmission sites. An infected person sheds fungal cells onto the wet surface, and the organisms survive long enough for the next barefoot walker to pick them up. The infection often starts as athlete’s foot between the toes and later spreads to the nails.
Your own shoes can also be the problem. Fungi flourish inside footwear that stays warm and moist, especially if you wear the same pair daily without letting them dry out. Ill-fitting shoes compound the risk by creating repeated micro-trauma to the nails while trapping moisture. Research on predisposing habits found that barefoot walking in communal areas, wearing tight shoes, and frequent exposure to detergents or water (which softens and weakens the nail) all increase vulnerability. Nail biting was also identified as a contributing factor for fingernail infections.
Why Some People Are More Susceptible
Age is the strongest predictor. Nail growth slows steadily after age 25, declining by about 0.5% per year. By your 60s or 70s, your nails grow significantly slower than they did in your 20s. Since a healthy, fast-growing nail naturally pushes out minor infections before they take hold, slower growth gives fungi more time to establish themselves. Older adults also tend to have reduced blood flow to the extremities and a less responsive immune system, both of which make it harder for the body to fight off a fungal invader in the nail bed.
Diabetes creates a particularly high-risk scenario. Poor blood sugar control damages small blood vessels in the feet and impairs the immune cells that would normally attack fungal organisms. High blood sugar also causes chemical changes in nail proteins themselves. Sugar molecules bind to keratin and other nail proteins, and there is evidence that these modified proteins actually give fungi better grip, essentially making it easier for the organisms to latch onto the nail. People with diabetes also tend to have reduced sensation in their feet, meaning minor nail injuries that create entry points go unnoticed.
Other conditions that raise your risk include peripheral artery disease (which reduces blood flow to the toes), obesity, smoking, and any form of immune suppression. Psoriasis and other skin conditions that affect the nails can also create structural vulnerabilities that fungi exploit. Men are affected more often than women, though the reasons for this aren’t fully understood.
Toenails vs. Fingernails
Toenails are far more commonly infected than fingernails. The reasons are straightforward: toes spend most of the day enclosed in shoes, creating the warm, dark, humid environment fungi prefer. Toenails also grow about three times slower than fingernails (roughly 1 millimeter per month compared to 3 millimeters), giving infections much more time to take root. Blood circulation to the toes is weaker than to the fingers, which means a less robust immune response at the site.
Fingernail infections do occur but tend to involve different organisms. Yeast infections are more common in fingernails, particularly in people whose hands are frequently wet, like dishwashers, bartenders, or healthcare workers who wash their hands many times a day. Repeated water exposure softens the cuticle and the seal between the nail and surrounding skin, creating openings for yeast to enter.
Why It Spreads and Persists
Nail fungus is notoriously stubborn because the nail plate itself has very limited blood supply. Your immune system fights infections primarily through blood-borne white blood cells, but these cells have poor access to the hard nail tissue where the fungus lives. This makes the nail something of an immune-privileged zone, a place where the body’s defenses have trouble reaching.
The infection can spread from nail to nail on the same foot, to the other foot, and to other people in the household through shared surfaces, towels, or nail clippers. It can also cycle back and forth between the skin and nails. You might successfully treat athlete’s foot between your toes, only to have the fungus still living in a toenail reinfect the surrounding skin weeks later. This is one reason treatment courses for nail fungus typically last months: the medication needs to remain active long enough for the entire infected portion of the nail to grow out and be replaced by healthy tissue.
Left untreated, a fungal nail can thicken to the point where it presses painfully against the inside of your shoe, making walking uncomfortable. The distorted, crumbly nail can also create breaks in the surrounding skin that allow bacteria to enter, potentially leading to secondary skin infections. This is a particular concern for people with diabetes, where even minor foot infections can escalate quickly.
Getting an Accurate Diagnosis
Not every thick, discolored nail is fungal. Psoriasis, repeated trauma (common in runners), and simple aging can all produce nail changes that look similar. A standard screening test involves dissolving a nail clipping in a chemical solution and examining it under a microscope for fungal structures. This test is highly reliable when it’s positive (95% specificity), but it misses infections roughly 40% of the time, so a negative result doesn’t rule fungus out.
Fungal cultures, where a nail sample is placed on a growth medium to see what develops, are even more specific (99%) but also miss a significant number of true infections, with sensitivity around 56%. Because no single test catches every case, doctors sometimes need to repeat testing or combine methods to confirm the diagnosis before starting treatment that may last several months.