Toenail fungus is caused by fungi that thrive in warm, moist environments and feed on keratin, the protein that makes up your nails. Dermatophytes, a group of fungi specially adapted to break down keratin, account for about 90% of toenail infections. The condition affects up to 20% of people at some point in their lives, making it the most common nail disorder in adults.
The Fungi Behind the Infection
The single biggest culprit is a dermatophyte called Trichophyton rubrum. This organism also causes athlete’s foot, and the two infections frequently go hand in hand. A second dermatophyte, Trichophyton mentagrophytes, is the next most common cause. Together, these two species are responsible for the vast majority of cases in North America and Europe.
Yeasts and molds can also infect toenails, though far less often. Candida albicans, the same yeast behind oral thrush and vaginal yeast infections, is the most common yeast involved. Various environmental molds, including Aspergillus and Fusarium species, occasionally cause infections too. These nondermatophyte molds are more common in tropical climates and can be harder to treat because standard antifungal medications primarily target dermatophytes.
How Fungi Get Into the Nail
Fungi don’t bore through healthy, intact nail from the top. In the most common pattern, the fungus first colonizes the skin around your toes (often as athlete’s foot) and then creeps under the free edge of the nail at the tip. From there, it spreads backward along the nail bed, the soft tissue underneath. This is why infections typically start at the tip or sides of the nail and gradually work their way toward the cuticle.
A less common route is direct invasion of the nail surface, which shows up as white, chalky patches on top of the nail rather than the typical yellowing underneath. In rare cases, the fungus enters through the cuticle end and works forward, a pattern more often seen in people with weakened immune systems.
Moisture plays a key role in all of these pathways. When the nail plate stays damp for long periods, it softens, making it physically easier for fungi to penetrate deeper into the nail bed. Any damage to the nail, whether from stubbing your toe, tight shoes, or repeated minor trauma from running, creates additional entry points.
Footwear and Daily Habits
Fungus thrives in environments that are dark, warm, and moist, which is a near-perfect description of the inside of a closed shoe. Heavy leather work boots, steel-toed safety shoes, and insulated winter boots are especially problematic because they trap heat and don’t breathe. Pair that with thick cotton socks that absorb sweat and hold it against the nail bed, and you’re essentially incubating fungal spores for eight to ten hours a day.
People who wear the same pair of boots daily without letting them dry out between uses face an even higher risk. The interior never fully dries, so fungus from the skin has a persistent warm, humid environment to multiply in. Over time, it migrates from the surrounding skin into the nails. This is why toenail fungus is disproportionately common among people in trades, the military, and athletes who train in closed shoes for hours at a stretch.
Walking barefoot in communal wet areas like gym showers, locker rooms, and pool decks is another well-known exposure route. The fungi shed by other people’s feet survive on damp surfaces and can transfer to your skin on contact. Shared nail clippers and pedicure tools that haven’t been properly sterilized carry the same risk.
Age and Circulation
Toenail fungus becomes significantly more common with age. Nails grow more slowly as you get older, giving fungi more time to establish an infection before the nail can grow out and shed the affected area. Older nails also tend to be thicker and drier, developing microscopic cracks that serve as entry points. Decades of cumulative exposure to the fungus in shoes and public spaces add up as well.
Reduced blood flow to the feet compounds the problem. When circulation is poor, the immune system has a harder time detecting and fighting off infections in the toes. This is one reason toenail fungus is so stubborn once it takes hold: even in younger, healthy people, blood flow to the nail bed is relatively limited compared to other parts of the body.
Diabetes, Immunity, and Other Medical Risks
Almost a third of people with diabetes develop toenail fungus. High blood sugar impairs the immune response and can reduce sensation in the feet, meaning infections go unnoticed longer. For people with diabetes, a toenail infection isn’t just cosmetic. It can become a significant threat to the limb if the fungus creates breaks in the skin that allow bacteria in.
Immunosuppression from any cause raises risk. About one in four people living with HIV develop toenail fungus. Organ transplant recipients taking immunosuppressive medications, people on long-term corticosteroids, and those undergoing chemotherapy are all more vulnerable. In these groups, infections tend to be more widespread, progress faster, and respond less readily to treatment.
The relationship between peripheral vascular disease and toenail fungus is still being sorted out. Some research links venous insufficiency (poor blood return from the legs) to higher rates of infection, while the connection to arterial disease is less clear. What is clear is that anything reducing blood flow or immune surveillance in the feet creates a more hospitable environment for fungi.
How to Tell It Apart From Other Nail Problems
Not every thick, discolored toenail is fungal. Nail psoriasis is the condition most commonly confused with toenail fungus, and the two can even occur on the same nail simultaneously. A few patterns help distinguish them. Fungal infections tend to start at the tip and progress slowly in one direction until they reach a certain level in the nail, then stabilize. Psoriasis tends to fluctuate, with flare-ups and periods of improvement.
Psoriasis more often produces tiny pits on the nail surface, salmon-colored spots underneath the nail, and involvement of multiple fingernails alongside toenails. Fungal infections rarely cause pitting and are far more common on toes than fingers. The distinction matters because the treatments are fundamentally different: antifungal therapy for fungus, anti-inflammatory or immune-modulating therapy for psoriasis. Using the wrong approach won’t help and can make things worse.
Repeated trauma to the nail, common in runners and hikers, can also mimic fungal infection by causing thickening, discoloration, and separation of the nail from the bed. A lab test that identifies fungus in a nail clipping is the only way to confirm the diagnosis with certainty.
Why Toenails Are More Vulnerable Than Fingernails
Toenails are affected far more often than fingernails, and the reasons are straightforward. Toes spend most of the day enclosed in shoes, sitting in exactly the warm, dark, humid conditions fungi prefer. Fingernails are exposed to air and light, which naturally inhibit fungal growth. Toenails also grow roughly two to three times slower than fingernails, giving an infection more time to take root before the nail grows out. And because toes are farther from the heart, they receive less blood flow, meaning a weaker local immune defense. All of these factors stack on top of each other, which is why toenail fungus is so much more common and so much harder to clear than fingernail infections.