Toenail fungus is caused by microscopic fungi that slip through tiny cracks or cuts in your skin and settle between your toenail and the nail bed underneath. Once there, the fungi feed on keratin, the tough protein that makes your nails hard, gradually breaking down the nail structure. About 5.5% of the global population has a toenail fungal infection at any given time, and hospital-based studies suggest the real number may be closer to one in four people at some point in their lives.
The Fungi Behind the Infection
A group of molds called dermatophytes cause 80% to 90% of all toenail fungal infections. These organisms evolved specifically to digest keratin, which is why they target nails, skin, and hair rather than other parts of the body. They don’t need sunlight or deep tissue to survive. They thrive in the warm, slightly damp layer between your nail plate and the soft tissue below it.
Yeast accounts for another 2% to 11% of cases. Yeast-related toenail infections are more common on fingernails and in people who frequently have wet hands or feet. A small percentage of infections come from non-dermatophyte molds found in soil and decaying plant material. These are rarer and sometimes harder to treat because standard antifungal medications are designed with dermatophytes in mind.
How the Fungus Gets In
Fungi don’t bore through a healthy, intact nail. They need an entry point. That’s usually a small crack, a minor separation between the nail and the nail bed, or damage from stubbing your toe or wearing tight shoes. Even a tiny cut along the cuticle or the skin beside the nail can be enough. Athlete’s foot, which is caused by the same family of dermatophytes, often serves as a launching pad. The fungus spreads from the skin around the toes to the nail itself.
Once inside, the fungus begins breaking down keratin to fuel its growth. This is why infected nails become thick, crumbly, and discolored. The nail isn’t just stained on the surface. It’s being slowly digested from the underside, which is why topical treatments applied to the top of the nail often struggle to reach the infection.
Why Toenails Are More Vulnerable Than Fingernails
Toenails get infected far more often than fingernails, and the reasons are straightforward. Your feet spend hours sealed inside shoes, creating a warm, moist environment that fungi love. Blood circulation to the toes is weaker than to the fingers, which means your immune system is slower to detect and fight off invaders in that area. Toenails also grow much more slowly than fingernails, giving the fungus more time to establish itself before the nail can grow out and shed the infected portion.
Age amplifies every one of these factors. As you get older, your nails grow even more slowly, become drier and more prone to cracking, and circulation to the extremities decreases further. This is why toenail fungus is significantly more common in older adults.
Risk Factors That Increase Your Chances
Some people are more susceptible than others, and it comes down to a combination of environment, habits, and biology.
- Occlusive footwear: Shoes that trap heat and moisture, particularly athletic shoes worn for long periods, create ideal conditions. Research on collegiate runners found they face high infection risk because they train in enclosed shoes, use shared locker rooms, experience repeated minor foot trauma, and go through periods of reduced immune function from intense training.
- Shared wet surfaces: Communal showers, pool decks, and locker room floors are common places to pick up fungal spores. Walking barefoot in these areas is one of the most well-established transmission routes.
- Nail trauma: Repeated pressure on the toenails from running, hiking, or ill-fitting shoes creates micro-damage that opens the door to infection.
- Weakened immune system: Diabetes, peripheral vascular disease, and conditions or medications that suppress immune function all make it easier for fungi to take hold and harder for your body to clear them.
- Pre-existing athlete’s foot: The same dermatophytes cause both conditions. If you already have a fungal skin infection on your feet, the risk of it spreading to a nail is high.
- Poor shoe hygiene: Limited washing of sneakers and reusing sweaty socks contribute to repeated fungal exposure. Research suggests washing shoes and socks at 60°C (140°F) is needed to kill fungal spores.
Where You Pick It Up
Fungal spores are remarkably durable. They can survive on surfaces for months, waiting for contact with a suitable host. Locker room floors and shared shower areas are the classic sources, but your own shoes can be just as problematic. If your shoes harbor spores from a previous infection or environmental exposure, you’re re-introducing fungi to your feet every time you lace up. This is one reason toenail fungus is so notorious for coming back after treatment.
Nail salons are another source when tools aren’t properly sterilized between clients. The metal instruments used to push back cuticles and file nails can carry spores from one person’s infected nail to the next person’s healthy one. Sharing nail clippers or files at home carries the same risk on a smaller scale.
What Happens If It Spreads
Toenail fungus itself isn’t dangerous in most healthy people, but it doesn’t resolve on its own. Left untreated, it tends to worsen and spread to other nails. The nail can become so thick and distorted that it presses painfully against the inside of your shoe or makes walking uncomfortable.
The more serious concern is that damaged, fungal nails create openings in the skin that bacteria can exploit. Cellulitis, a potentially serious skin infection caused by streptococcus or staphylococcus bacteria, can develop when bacteria enter through cracked or broken skin around an infected nail. The Mayo Clinic specifically lists athlete’s foot and toenail fungal infections as entry points for the bacteria that cause cellulitis. For people with diabetes or compromised circulation, this risk is especially relevant because their bodies are slower to fight bacterial infections in the feet.
Why It Keeps Coming Back
Recurrence is one of the most frustrating aspects of toenail fungus, and it circles back to the causes. Even after successful treatment, the same environmental exposures, shoe habits, and biological vulnerabilities that allowed the first infection remain. Spores can linger in old shoes, shower floors, and bedding. If you had athlete’s foot alongside the nail infection and only treated the nail, the skin infection can seed a new nail infection within months.
The slow growth rate of toenails also plays a role. A big toenail takes 12 to 18 months to grow out completely, meaning treatment needs to continue long after the nail looks healthy. Stopping early leaves residual fungus in the nail bed that can re-establish itself in the new nail growing in above it.