Fingernail fungus is caused by microscopic organisms, most commonly yeasts and dermatophytes, that invade the hard protein (keratin) your nails are made of. About 10% of people worldwide have a fungal nail infection, though toenails are affected seven to ten times more often than fingernails. When fingernails are involved, the cause is usually different from what infects toenails, and the risk factors are more closely tied to what your hands are exposed to every day.
The Fungi Behind Fingernail Infections
Fingernail and toenail fungus are not the same disease. On toenails, a skin-loving fungus called a dermatophyte is almost always the culprit. On fingernails, yeasts dominate. A large laboratory study that profiled fingernail infections found that yeasts accounted for 86% of all fungal species identified. The most common was Candida parapsilosis, responsible for roughly a third of cases, followed by Candida albicans at about 19%. Dermatophytes made up less than 9% of fingernail infections, with nearly all of those being a single species, Trichophyton rubrum.
A smaller percentage of fingernail infections come from environmental molds like Fusarium and Aspergillus, each accounting for only about 1.5 to 2% of cases. These mold infections are harder to treat and sometimes get misidentified as one of the more common types, which is one reason lab testing matters before starting treatment.
People with weakened immune systems, including those with HIV or chronic mucocutaneous candidiasis, are especially prone to yeast-driven fingernail infections. Women develop Candida fingernail infections more often than men, likely because of differences in hand exposure to water and cleaning products.
How Fungi Break Into the Nail
Your nails are made of keratin, a tough structural protein held together by dense networks of chemical bonds, especially sulfur bridges called disulfide bonds. Healthy, intact nails are remarkably resistant to infection because this tightly cross-linked structure is difficult for most organisms to penetrate.
Fungi that infect nails have evolved a specific strategy to get through this barrier. First, they produce sulfite and specialized enzymes that break the sulfur bridges holding keratin together. Once those bonds are severed, the protein structure loosens and unfolds, exposing it to a second wave of enzymes (proteases and keratinases) that digest the keratin itself. The fungus essentially dismantles the nail from the inside out, using the broken-down protein as food. This is why infected nails become thick, crumbly, and discolored: the structural material is literally being consumed.
Any damage to the nail gives fungi a head start. Trauma to the nail plate, including tiny injuries from aggressive manicures, creates entry points that bypass the nail’s natural defenses. Once inside, fungi can establish themselves in the nail bed where they’re protected from topical treatments and the immune system alike.
Water Exposure and Occupation
Prolonged or repeated contact with water is the single biggest environmental risk factor for fingernail fungus. Jobs that keep hands wet for long stretches, such as dishwashing, food preparation, cleaning, bartending, and healthcare work, soften the nail and the surrounding skin. This weakens the seal between the nail and the nail bed, letting yeast organisms (particularly Candida species) gain a foothold. Frequent handwashing carries a similar risk, which is why healthcare workers develop fingernail infections more often than the general population.
Humid environments compound the problem. Fungi thrive in warm, moist conditions, and hands that are repeatedly wet without fully drying create an ideal growth environment right at the nail margin.
Nail Salons and Shared Tools
Nail salons are a well-documented source of fungal transmission. Clippers, files, cuticle pushers, and other metal tools can carry fungal spores from one client to the next. Studies examining salon tools have found Candida species and other fungi on nail care instruments, and clippers in particular tend to receive the least thorough sterilization of any salon tool. Even when cleaning practices are in place, acceptable levels of sterilization are rarely achieved.
The cosmetic products themselves can also harbor fungi. Nail products with high water content provide the sugars, proteins, and moisture that microbes need to survive. One study found that Trichophyton rubrum can survive inside nail polish for up to 60 days at room temperature. Sharing nail polish between people, whether at a salon or at home, can move fungi from one person’s nails to another’s.
Manicures also create micro-trauma to the nail and cuticle. Aggressive pushing or cutting of the cuticle removes a natural barrier that helps seal the nail root against infection. The combination of tiny wounds and contaminated tools makes salons a particularly efficient route of transmission.
Household and Environmental Spread
Fungi don’t just live on people. Dermatophytes persist on a surprising range of surfaces: cleaning supplies like mops and scrubbers, vacuum cleaners, bedding, and clothing. One study found clinically relevant fungi on 88% of household cleaning supplies sampled. Other research has isolated common nail-infecting species from garden soil and beach sand.
Sharing personal items increases risk. Nail tools, towels, and even footwear can carry fungal spores between household members. Socks and clothing can harbor dermatophytes even after regular laundering. Pets are another overlooked source. Cats, dogs, and small rodents like guinea pigs can carry fungal species that infect human nails, and pet ownership has been identified as an independent risk factor for nail fungus.
Diabetes and Circulation Problems
Diabetes substantially raises the risk of nail fungus through several overlapping mechanisms. Persistently high blood sugar damages small blood vessels, reducing circulation to the fingers and toes. This impairs the delivery of immune cells to the nail bed. At the same time, high glucose levels directly weaken the function of neutrophils and other white blood cells that would normally fight off fungal invaders.
There is also a molecular component. Chronic high blood sugar leads to the buildup of compounds called advanced glycation end-products (AGEs) in tissue. These AGEs interfere with the immune system’s ability to recognize and respond to fungal pathogens. They may also make the nail itself more hospitable to fungi by altering keratin proteins in ways that give fungal organisms better surfaces to attach to. For people with diabetes, a fungal nail infection is not just a cosmetic issue: it can serve as an entry point for bacterial infections and contribute to more serious complications.
Peripheral arterial disease, even without diabetes, produces a similar effect. Reduced blood flow to the extremities means slower nail growth and a weaker local immune response, both of which favor fungal colonization.
Aging and Slower Nail Growth
Fingernail fungus becomes more common with age, and the reasons are largely mechanical. Fingernails normally grow at about 3 millimeters per month, but starting around age 25, that rate drops by roughly 0.5% per year. By your 60s or 70s, your nails are growing meaningfully slower than they did in your 20s.
This matters because nail growth is one of your body’s natural defenses against infection. A fast-growing nail pushes fungal organisms outward before they can establish a deep infection. Slower growth gives fungi more time to invade and spread. Combine that with the reduced blood circulation that comes with aging and the accumulated effects of decades of micro-trauma, and older nails become significantly more vulnerable. Male sex, smoking, and a family history of nail infections further increase risk in older adults.
Weakened Immune Systems
Any condition or medication that suppresses the immune system opens the door to fungal nail infections. HIV is one of the strongest risk factors: roughly 1 in 4 HIV-positive individuals develop nail fungus. Cancer, organ transplant recipients taking immunosuppressive drugs, and people on long-term corticosteroids are also at elevated risk.
In immunocompromised people, fingernail infections behave differently. Multiple fingernails are more likely to be affected at once, the infections tend to be more severe, and yeast organisms (rather than dermatophytes) are the more common cause. In someone with a healthy immune system, having more than one fingernail infected without any toenail involvement is unusual enough that it may prompt a doctor to look for an underlying immune problem.
How Fingernail Fungus Is Confirmed
Fingernail fungus can look identical to nail psoriasis, eczema-related nail damage, or simple trauma. Visual inspection alone is unreliable, which is why lab testing is important before committing to months of treatment. The standard first step is a KOH preparation: a clinician clips a portion of the affected nail, scrapes debris from under it, and treats the sample with potassium hydroxide to dissolve everything except fungal elements. Under a microscope, fungal threads or yeast cells become visible.
A fungal culture takes longer, usually several weeks, but identifies the exact species causing the infection. This distinction matters for treatment because dermatophyte infections, yeast infections, and mold infections respond to different medications. In cases where results are unclear, a small nail biopsy can provide a definitive answer.