"Where did this come from?" That's one of the first questions patients ask me when I diagnose nail fungus. And I get it. There's something unsettling about an infection you didn't see coming. The truth is, the fungi responsible for nail infections are remarkably common organisms that exist in everyday environments. Understanding where they live and how they get into your nails is the first step toward preventing and treating the problem.
The Organisms Behind Nail Fungus
Nail fungus (onychomycosis) is caused by microscopic fungi that feed on keratin, the protein that makes up your nails, skin, and hair. About 85% of nail fungus cases are caused by a group of fungi called dermatophytes. The most common species is Trichophyton rubrum, followed by T. interdigitale. These organisms are specifically adapted to living in human tissue, which is why they're so effective at causing infection.
In some cases, non-dermatophyte molds and yeast (particularly Candida species) can also infect nails, though this is less common. Identifying the specific organism matters because it can influence which treatment approach works best.
How Nail Fungus Gets In
Here's the part most people find surprising: fungi don't need a dramatic injury to infect your nails. All it takes is a microscopic opening in or around the nail. A tiny crack in the skin near the nail edge, a small separation between the nail and nail bed, even a hangnail or minor cut from an aggressive pedicure can provide enough access.
Once fungi find that entry point, they settle into the warm, dark, slightly moist environment beneath the nail plate. And that's an ideal home for them. The nail acts as a shield, protecting the fungal colony from your immune system and from most topical treatments. This is exactly why nail fungus is so persistent and why over-the-counter remedies rarely work: the infection is literally hiding behind a barrier your body can't easily breach.
The Five Most Common Ways People Get Nail Fungus
1. Walking barefoot in communal wet areas. Pool decks, gym locker rooms, shared showers, and spa floors are prime real estate for fungal organisms. These warm, moist surfaces can harbor fungi for extended periods. One barefoot walk across a contaminated locker room floor is all it takes.
2. Athlete's foot spreading to the nails. This is actually the most common pathway I see in my practice, and it's one that patients often overlook. Athlete's foot (tinea pedis) is caused by the same fungal organisms that cause nail infections. That flaky, itchy, cracking skin between your toes or on the soles of your feet? It's a fungal reservoir sitting right next to your toenails. If left untreated, the fungus can and will migrate into the nail. In fact, the majority of patients with toenail fungus also have (or have had) athlete's foot. I always tell patients: treating athlete's foot is also nail fungus prevention.
3. Sharing contaminated personal items. Nail clippers, towels, shoes, and nail files can all harbor fungal organisms. If someone in your household has a nail infection and you're sharing these items without proper disinfection, you're at risk. Fungi can survive on surfaces for extended periods.
4. Prolonged moisture exposure. People whose feet spend long hours in damp conditions are at significantly higher risk. This includes athletes, people who wear work boots all day, and anyone whose job involves frequent hand washing or wet environments. The constant moisture softens the nail and surrounding skin, creating easy entry points for fungi.
5. Direct contact with an infected person. Nail fungus is contagious. Touching someone's infected nails or the skin around them can transfer fungal organisms to your own nails or skin. This is particularly common among family members living in the same household.
Who's Most at Risk?
While anyone can develop nail fungus, certain factors significantly increase your likelihood.
Age Is the Biggest Risk Factor
Nail fungus becomes dramatically more common as we age. By age 70, roughly half of adults have some degree of onychomycosis. There are several reasons for this: nails grow more slowly with age (giving fungi more time to establish), blood circulation to the extremities decreases, and the immune system becomes less effective at fighting infections. Nails also become drier and more brittle over time, creating more microscopic entry points for fungi.
Children, interestingly, rarely get nail fungus. As Dr. Shari Lipner at Weill Cornell has noted, when a child does develop it, it almost always means a parent in the household has it too.
Medical Conditions That Increase Risk
Several health conditions create an environment where nail fungus thrives:
- Diabetes reduces blood flow to the feet and can impair the immune response. Diabetic patients also need to be more vigilant about treating nail fungus because it can create openings that lead to secondary bacterial infections.
- Peripheral vascular disease and poor circulation for any reason reduce the body's ability to fight infections in the extremities.
- Psoriasis can cause nail changes that mimic (and coexist with) fungal infection, and damaged psoriatic nails are more susceptible to fungal colonization.
- Immunosuppression from conditions like HIV, cancer treatment, or immunosuppressive medications makes fungal infections more likely and harder to clear.
- Obesity can make it difficult to reach and care for toenails properly, and is associated with increased sweating.
- Inflammatory bowel disease has been linked to higher rates of fungal infections.
Lifestyle and Environmental Factors
- Athletes (especially runners and swimmers) are at elevated risk due to repetitive nail trauma, communal shower exposure, and prolonged sweating in closed-toe shoes
- Wearing tight or non-breathable footwear traps moisture against the feet
- Excessive sweating (hyperhidrosis) creates a persistently moist environment
- Prior nail injury or nail surgery provides entry points for fungi
- Keeping nails too long allows debris (and fungi) to accumulate underneath
- Using improperly sanitized salon equipment is a commonly overlooked risk factor
Family History Matters
There appears to be a genetic component to susceptibility. If your parents or siblings have dealt with persistent nail fungus, you may be more prone to it yourself. The reasons aren't entirely clear, but likely involve inherited differences in immune response and skin barrier function.
The Athlete's Foot Connection Deserves Extra Attention
I want to emphasize this point because it's so commonly missed: athlete's foot and nail fungus are caused by the same organisms and are essentially the same infection in different locations. About 34% of people with athlete's foot will eventually develop toenail fungus. And the odds ratio for developing cellulitis (a potentially serious bacterial skin infection) in patients with tinea pedis is 2.4 to 3.2, depending on the type.
If you have cracked, scaly, or peeling skin on your feet, especially between the toes, or itching and burning on the soles... that's athlete's foot, and it's actively putting your nails at risk. Treating it aggressively isn't just about comfortable feet. It's about preventing a much more stubborn and longer-lasting nail infection.
Why Early Treatment Matters
Understanding what causes nail fungus leads naturally to the most important takeaway: this isn't a condition that improves with time. The fungi are living organisms that will continue to grow, spread deeper into the nail, and potentially infect other nails and household members unless you treat them.
The encouraging news is that prescription dual-therapy approaches (combining a topical antifungal solution that can penetrate the nail plate with oral terbinafine that reaches the nail bed through the bloodstream) achieve cure rates exceeding 85%. That's a dramatic improvement over OTC products, which hover below 10% because they simply cannot get past the nail barrier.
If you're seeing the signs of nail fungus or you know you have risk factors, a dermatologist-designed prescription treatment that attacks the infection from both sides of the nail is the most effective path forward. And if you have athlete's foot, treat that too. Your nails will thank you.
