Onychomycosis is a fungal infection of the nails that causes the nail plate to become thick, discolored, and brittle. While not life-threatening, this widespread condition can cause discomfort, embarrassment, and in some cases lead to more serious health complications, especially in people with certain medical conditions.
How Common Is Onychomycosis?
Onychomycosis is one of the most common nail problems people face around the world. Medical experts estimate that this fungal infection affects roughly 1 in 10 people in the general population. However, the likelihood of developing onychomycosis increases significantly with age. Studies show that approximately 20% of people over 60 years old have this condition, and the number jumps dramatically to about 50% of those over 70 years old.[1][3]
The infection is more common in adults than in children and adolescents, where the prevalence is estimated at only 0.44%. Men tend to develop onychomycosis more frequently than women. The condition primarily affects toenails rather than fingernails, with toenails being affected about 10 times more often. This difference exists because toenails grow more slowly, receive less blood supply, and are more frequently confined in dark, moist environments like shoes.[4][7]
Onychomycosis accounts for roughly one-third of all fungal skin infections and represents about half of all nail diseases. The increasing incidence of this condition over recent decades has been attributed to factors such as occlusive footwear, exposure in locker rooms and public pools, and the worldwide spread of different fungal strains.[10]
What Causes Onychomycosis?
Onychomycosis develops when fungi invade the nail unit, which includes the nail plate, nail bed, and nail matrix. The infection occurs when fungi get between the toenail and the tissue right underneath it, usually through a crack or cut in the toe.[1]
The most common cause is a type of mold called a dermatophyte, which is a fungal microorganism too tiny to see with the naked eye. Dermatophytes need a protein called keratin to grow, which is the main structural material that makes nails hard. When dermatophytes cause nail fungus, the condition is specifically called tinea unguium. These organisms are responsible for approximately 90% of toenail fungal infections and about 50% of fingernail infections.[1][3]
The most frequently identified dermatophyte in onychomycosis cases is Trichophyton rubrum. Other dermatophytes that can cause the infection include Trichophyton mentagrophytes and Epidermophyton floccosum.[3][4]
However, dermatophytes are not the only culprits. Yeasts, particularly Candida albicans, account for about 2% of onychomycosis cases, especially in fingernails. People with chronic mucocutaneous candidiasis or weakened immune systems are more susceptible to yeast-caused nail infections. Additionally, nondermatophyte molds such as Fusarium, Aspergillus, Acremonium, and Scopulariopsis brevicaulis can cause nail infections, accounting for roughly 8% of cases. These molds are cultured primarily from toenails.[3][7]
Research has shown that nearly 39% of onychomycosis infections are actually mixed infections, meaning they are caused by a combination of dermatophytes plus nondermatophyte molds or yeasts. This complexity can make both diagnosis and treatment more challenging.[15]
How Does Onychomycosis Spread?
Many types of nail fungus, including tinea unguium, are quite contagious. You can spread the fungus to someone else through direct skin-to-skin contact. You can also contract the infection by touching surfaces that have been contaminated with the fungus.[1][8]
Nail fungi thrive in warm, moist, and dark environments. Common ways people acquire toenail fungus include walking barefoot around the perimeters of swimming pools, using public locker rooms or showers without protective footwear, and walking barefoot in other public areas. Sharing objects like towels, bed sheets, or even nail clippers with an infected person can also transmit the fungus.[1][8]
While toenail fungus usually doesn’t spread beyond the toe, it can sometimes migrate to other body areas. Some dermatophyte fungi spread easily to the skin, since skin and scalp also contain keratin. When dermatophyte fungi affect the skin, the resulting condition is called ringworm. Toenail fungus may spread to other toenails, to the skin between the toes (causing athlete’s foot), to the groin area (causing jock itch), or even to the scalp.[1][16]
Who Is at Higher Risk?
While anyone can develop onychomycosis, certain groups of people face a higher risk. Age is one of the most significant risk factors. As people grow older, several changes occur that increase vulnerability: peripheral blood circulation decreases, immune function becomes less optimal, nail growth slows down, and there is simply longer exposure to pathogenic fungi over a lifetime.[15]
People with diabetes have a significantly elevated risk, with studies showing the risk is 1.9 to 2.8 times higher compared to the general population. Those with compromised immune systems, whether from autoimmune disorders, HIV infection, or immunosuppressive treatments, are also more susceptible. In fact, proximal subungual onychomycosis (a form that starts at the base of the nail) is considered a clinical marker of HIV infection.[1][4][10]
Other medical conditions that increase risk include poor blood circulation due to peripheral vascular disease, psoriasis, and hyperhidrosis (a disorder causing excessive sweating). People with a history of nail injuries are also at higher risk, as damaged nails provide easier entry points for fungi.[1][15]
Having athlete’s foot (tinea pedis) substantially increases the likelihood of developing nail fungus, and some degree of athlete’s foot is almost always present alongside toenail fungus. Wearing tight-fitting shoes and socks, using occlusive footwear that doesn’t allow feet to breathe, and participating in contact sports like wrestling can all contribute to infection risk. Smoking tobacco also appears to be a risk factor. Interestingly, susceptibility may run in families, with some research suggesting it can occur in an autosomal dominant pattern.[1][4][10][15]
What Are the Symptoms?
The symptoms of onychomycosis can vary depending on the type and severity of infection, but there are several characteristic signs. The most common symptom is a change in the nail’s appearance. Infected nails typically become discolored, appearing white, yellow, brown, or sometimes even black or green. The nail may develop white spots and streaks or appear chalky or cloudy in certain areas.[1][2][7]
As the fungal infection progresses, the nail often thickens and may become misshapen. The nail can become brittle, crumbly, or ragged, with pieces breaking off or coming away from the finger or toe completely. Another common sign is onycholysis, which means the nail separates from the nail bed, leaving a space between the nail and the skin underneath. Debris composed of keratin and fungal material may accumulate beneath the nail. In some cases, the nail may emit a foul smell.[1][2][7][15]
Typically, toenail fungus is not painful, though the appearance can be unsightly and cause embarrassment. However, as the disease advances and the nail becomes severely thickened and distorted, it may begin to cause discomfort, pain, or interfere with walking, standing, and exercising. The skin underneath and around the nail can become inflamed and painful if left untreated.[1][7][15]
People with onychomycosis may experience significant emotional and social problems due to the nail’s appearance, particularly when fingernails are affected since hands are always visible. This can lead to loss of self-esteem and inhibited social interaction.[4][5][7]
How to Prevent Onychomycosis
Preventing onychomycosis involves several practical strategies focused on keeping feet dry, maintaining good hygiene, and avoiding exposure to fungi. Since fungi thrive in warm, moist environments, keeping your feet clean and dry is fundamental. Wash your feet daily with soap and water, paying special attention to the spaces between your toes. After washing, ensure your feet are completely dry, especially between the toes, as trapped moisture creates an ideal environment for fungal growth.[17][24]
Proper nail care is essential. Keep your toenails trimmed short and filed down. Long nails can provide breeding grounds for fungi and are more likely to trap moisture and debris. If you have a fungal infection, use separate nail clippers for infected nails to prevent spreading the fungus to healthy nails.[17][18]
Footwear choices matter significantly. Select shoes made from breathable materials like leather or canvas that allow proper ventilation. Avoid tight-fitting shoes that don’t allow your feet to breathe, as these trap moisture and promote fungal growth. Wear sandals or flip-flops when using communal areas such as pools, locker rooms, showers, and gyms to avoid direct contact with contaminated surfaces.[1][17][24]
Change your socks regularly, ideally opting for moisture-wicking fabrics rather than cotton. Acrylic socks, though it may seem counterintuitive, are actually better at carrying moisture away from the skin than cotton. Consider using antifungal foot powder daily, and you can also apply antifungal sprays or powders inside your shoes to reduce moisture and inhibit fungal growth.[18][20][24]
Avoid walking barefoot in public places where the risk of exposure is high. Do not share personal items like towels, bed sheets, socks, or shoes with others. If you have athlete’s foot, treat it promptly, as it often coexists with and can lead to nail infections. For people who have successfully treated onychomycosis, not using old shoes after treatment may help decrease the risk of reinfection.[8][17][18]
Maintaining a strong immune system through a balanced diet rich in vitamins and minerals, regular exercise, and adequate sleep can also help your body fight off infections more effectively. For individuals with diabetes or other conditions affecting immune function or circulation, regular check-ups with a podiatrist to monitor nail health can help catch infections early before they become more serious.[18]
How the Body Changes with Onychomycosis
When onychomycosis develops, the infection triggers a series of changes in the nail unit’s normal structure and function. Understanding these changes helps explain why the condition is so persistent and challenging to treat.
The fungal organisms invade different parts of the nail depending on the type of infection. In the most common form, distal subungual onychomycosis, the fungi enter through the space between the nail and the skin at the tip of the toe (the hyponychium) and then migrate backward through the nail bed toward the base. As the fungi feed on keratin in the nail bed and the underside of the nail plate, they cause the nail to separate from the underlying tissue.[10]
The infection causes keratin and fungal debris to accumulate beneath the nail, leading to the characteristic thickening seen in infected nails. This buildup is called subungual hyperkeratosis. The nail plate itself becomes thicker, harder, and more brittle as it responds to the infection. The fungi also produce substances that cause discoloration, resulting in the yellow, white, or brown appearance of infected nails.[4][9]
In white superficial onychomycosis, fungi directly invade the superficial layers of the nail plate itself, forming well-defined white patches on the surface. These patches gradually spread and coalesce until they involve the entire nail plate, making it rough, soft, and crumbly.[10]
Once the nail has lifted away from the nail bed due to onycholysis, it typically won’t reattach. A new nail won’t grow from that separated portion of the nail bed. However, the nail continues to grow from the root at the base, which is why treatment focuses on allowing a new, healthy nail to replace the infected one over time. This process is extremely slow—fingernails require 3 to 6 months to completely regrow, while toenails need 12 to 18 months.[12][20]
The nail’s structure creates a significant barrier to treatment. The dense keratin of the nail plate makes it difficult for antifungal medications to penetrate deeply enough to reach the fungi residing in the nail bed. This is why topical treatments often have limited success and why oral medications that reach the nail through the bloodstream tend to be more effective.[13]
In the most advanced cases, total dystrophic onychomycosis can develop, representing the end stage of any subtype of the infection. At this point, the nail plate is completely destroyed and thickened beyond recognition, often causing significant discomfort and functional impairment.[10]
The presence of onychomycosis can also create a reservoir for fungi that may spread to other body areas or to other people. In individuals with compromised immunity, diabetes, or peripheral vascular disease, the infection can predispose them to more serious complications such as bacterial cellulitis of the lower leg, tissue damage, sepsis, osteomyelitis, and permanent nail loss.[3][9]
