Nail fungus is a fungal infection of the nail bed that causes thickening, discoloration, and crumbling of the affected nail. It’s one of the most common nail disorders, affecting an estimated 3 to 12% of the population, with toenails far more susceptible than fingernails. The infection is slow to develop, often starting as a small discolored spot at the nail’s edge before gradually spreading across the entire nail over months or years.
How Fungus Gets Into the Nail
The vast majority of nail fungus infections are caused by a group of fungi called dermatophytes, which feed exclusively on keratin, the protein that makes up your nails, skin, and hair. One species in particular is responsible for most cases. Dermatophytes account for about 90% of toenail infections and 50% of fingernail infections. Yeast causes around 2% of cases (mostly in fingernails), and various environmental molds make up about 8%.
The infection usually begins between the toes as athlete’s foot, then migrates toward the nail. Over time, the dark, warm, moist environment inside shoes, combined with small repetitive trauma to the nail (from walking, tight footwear, or stubbing), breaks the seal where the nail meets the skin at the fingertip. This gives the fungus an entry point. Once inside, it colonizes the nail bed underneath the nail plate, producing enzymes that break down keratin for fuel. The body responds with a low-grade inflammatory reaction that thickens the nail bed, which pushes the nail plate upward and eventually distorts it. Left alone, the infection spreads backward toward the base of the nail and can eventually damage the nail’s growth center.
What Nail Fungus Looks Like
Nail fungus doesn’t always look the same. There are three main patterns, and recognizing which one you have helps explain what’s happening beneath the surface.
- Distal and lateral subungual: The most common type. The tip and sides of the nail lift up, turn yellow or brown, and begin to crumble. This is the classic “thick, ugly toenail” most people picture.
- Superficial white: Flaky white patches and small pits appear on the top surface of the nail plate. The nail doesn’t thicken as dramatically at first, but the white areas can spread.
- Proximal subungual: Discoloration and thickening start near the base of the nail, close to the cuticle, and grow outward. This pattern is less common and can sometimes signal a weakened immune system.
In all three types, the nail may become brittle, develop a rough or ragged texture, and separate from the nail bed. Some people notice a mild odor. Pain is uncommon early on but can develop as the nail thickens enough to press against the inside of a shoe.
Who Is Most at Risk
Nail fungus becomes significantly more common with age, partly because nails grow more slowly in older adults (giving the fungus more time to establish itself) and partly because circulation to the feet declines. Men are affected more often than women.
People with diabetes face elevated risk for several overlapping reasons: reduced blood flow to the feet, diminished sensation that makes it easy to miss early nail trauma, and immune changes that make it harder to fight fungal colonization. Poor blood sugar control makes all of these worse. Obesity and peripheral vascular disease are independent risk factors as well. Having athlete’s foot is one of the strongest predictors, since the same fungus simply migrates from the skin to the nail. Anything that creates repeated micro-damage to the nail, from running to wearing tight shoes, also increases vulnerability.
How It’s Diagnosed
A thickened, discolored nail isn’t always fungus. Psoriasis, trauma, and other conditions can look similar, so confirming the diagnosis before starting treatment matters. Your doctor will typically clip or scrape a small sample from the affected nail and send it for testing.
The most common lab test dissolves the nail sample in a chemical solution and examines it under a microscope to look for fungal structures. This is quick but imperfect: its accuracy varies widely, catching anywhere from 34% to 93% of true infections depending on technique and sample quality. Fungal culture, where the sample is grown in a lab to identify the exact species, is more specific but slow (it can take weeks) and frequently returns false negatives. A tissue staining method has been shown to outperform both of these approaches in head-to-head comparisons. Some clinics now use dermoscopy, a magnified skin examination, to spot characteristic patterns like nail spikes and longitudinal striations that strongly suggest fungal infection before lab results come back.
Treatment Options and What to Expect
Nail fungus is notoriously stubborn. The nail plate acts as a physical shield that makes it difficult for medications to reach the infection in the nail bed underneath. Treatment choice depends largely on how much of the nail is involved.
Topical Treatments
For mild infections affecting less than half the nail, prescription topical solutions applied directly to the nail are the first option. These are safe and have minimal side effects, but their complete cure rates are modest. In clinical trials, the most effective topical option achieved complete cure in 15 to 18% of patients. A second option cleared the infection completely in 7 to 9% of cases, and a third, older nail lacquer cured 6 to 9%. “Complete cure” here means both eliminating the fungus and restoring a normal-looking nail, which is a high bar. Mycological cure rates (killing the fungus even if the nail still looks imperfect) are considerably higher, in the range of 29 to 55% depending on the product. Topical treatment requires daily application for about a year, and consistency matters enormously.
Oral Treatments
For moderate to severe infections, oral antifungal medications are more effective because they reach the nail bed through the bloodstream. These are typically taken daily for 6 to 12 weeks for toenails. Even after you stop taking the medication, the drug remains embedded in the nail and continues working as the nail grows out, so visible improvement continues for months after the course ends. It can take 9 to 12 months for a toenail to fully grow out and reveal the final result.
Oral antifungals do require liver function testing before and during treatment, since they can stress the liver in rare cases. Your doctor will order blood work to check liver enzymes before prescribing and may repeat the test partway through. Most people tolerate these medications without problems, but the monitoring is standard practice.
Why It Comes Back
Recurrence is one of the most frustrating aspects of nail fungus. Even after successful treatment, reinfection rates are high because the same risk factors that caused the original infection are still present: the same shoes, the same feet, the same environment. The fungus can also linger in surrounding skin (especially if athlete’s foot isn’t treated simultaneously) and recolonize the nail.
A few practical habits reduce your odds of reinfection:
- Wash and fully dry your feet daily, paying attention to the spaces between toes where fungus thrives.
- Change socks at least once a day, more often if your feet sweat heavily.
- Rotate shoes so each pair has time to dry out completely between wearings.
- Keep toenails trimmed short and clean, which reduces the area available for fungal colonization.
- Treat athlete’s foot promptly, since the same fungus causes both conditions and easily spreads from skin to nail.
- Wear sandals in shared wet areas like gym showers, pool decks, and locker rooms.
- Choose clean, licensed nail salons that sterilize instruments between clients if you get pedicures.
Complications Worth Knowing About
For most healthy people, nail fungus is a cosmetic nuisance and a source of mild discomfort. But for people with diabetes or compromised circulation, it can become genuinely dangerous. A thickened, distorted nail can press into surrounding skin and create small wounds. In someone with reduced sensation from nerve damage, those wounds may go unnoticed. Combined with poor blood flow and impaired immune function, this creates an entry point for bacterial infections that can escalate to serious skin infections or, in severe cases, threaten the foot itself. If you have diabetes, treating nail fungus early and maintaining careful foot hygiene is significantly more important than it is for the general population.