Nail fungus almost always starts with a tiny opening in or around the nail that lets fungal spores slip underneath. A small crack, a cut along the cuticle, or even microscopic damage from tight shoes is enough. Once fungi reach the warm, moist space between your nail and the skin beneath it, they begin feeding on keratin (the protein your nails are made of) and slowly spread deeper into the nail structure.
Where the Fungus Enters
The infection typically begins in the hyponychium, the strip of hardened skin right where your nail meets your fingertip or toe tip. From there, fungi spread to the nail bed and then upward into the nail plate itself. Most species target the underside of the nail first because that layer sits directly against the nail bed, giving the fungus easy access to moisture and nutrients.
Once attached, fungal threads (hyphae) penetrate individual nail cells and begin breaking apart the matrix that holds cells together. This loosens the nail’s internal structure, creating larger pores and gaps that let the infection move further inward. It’s a slow process. You can carry nail fungus for weeks or even months before you notice anything wrong.
What Causes the Infection
A group of fungi called dermatophytes causes over 75% of nail fungus cases. These organisms have evolved specifically to digest keratin, the same protein found in your skin, hair, and nails. They produce enzymes called keratinases that break the strong sulfur bonds holding keratin together, essentially dissolving the nail from the inside out.
Molds account for roughly 10% of cases, and yeast infections are uncommon but do occur, particularly in fingernails. People with weakened immune systems sometimes develop mixed infections involving more than one type of fungus, which can make treatment harder.
Why Toenails Are Hit Hardest
Toenails are far more susceptible than fingernails. The reason is simple: fungi thrive in dark, warm, moist environments, and the inside of a shoe checks every box. Heavy socks, especially cotton ones that absorb sweat and hold it against the nail, can essentially incubate fungal spores for eight to ten hours a day. That persistent moisture softens the nail plate, making it easier for fungi to invade deeper into the nail bed.
Your toes also receive less blood flow than your fingers, which means your immune system is slower to detect and fight off an infection there. Fingernails grow about twice as fast as toenails, so even if a fungus tries to take hold on a fingernail, fresh nail growth can sometimes outpace the infection. Toenails don’t have that advantage.
Common Entry Points and Triggers
Any break in the skin around your nail or damage to the nail itself creates an opportunity. Some of the most common scenarios include:
- Microtrauma from footwear. Shoes that press against the toenails, especially during running or hiking, cause tiny, repeated injuries to the nail plate and surrounding skin.
- Public wet surfaces. Shower floors at pools, gyms, and dorms are well-known transmission sites. Fungal spores shed from one person’s feet land on the warm, wet floor and transfer to the next barefoot person who walks through.
- Nail salon tools. Improperly sterilized clippers and files can carry spores from one client to the next.
- Athlete’s foot. A fungal skin infection on your foot can migrate to the nail. The same dermatophyte species cause both conditions, and leaving athlete’s foot untreated gives the fungus time to reach the nail.
Who Is Most Vulnerable
Nail fungus becomes more common with age, largely because nails grow more slowly, get drier, and develop more cracks over time. Reduced blood circulation in the feet also plays a role, since less blood flow means fewer immune cells patrolling the area.
People with diabetes face roughly three times the risk of developing nail fungus compared to people without it. Chronically elevated blood sugar weakens immune cell function, making it harder for the body to fight off fungal invaders. A cross-sectional study in Spain found that among people with diabetes, the risk climbed even higher for those with poor blood sugar control, cardiovascular disease, or a history of circulation problems in the legs. These individuals were also more likely to develop mixed infections involving multiple fungal species at once.
Other factors that raise your risk include psoriasis, a suppressed immune system (from medication or illness), heavy sweating, and a history of athlete’s foot. Working in jobs that keep your feet enclosed for long shifts, or that involve frequent hand-washing, can also set the stage.
What Early Nail Fungus Looks Like
The first visible sign is usually a small white or yellowish-brown spot near the tip of the nail. It’s easy to dismiss as a bruise or cosmetic blemish. At this stage, the fungus is just beginning to colonize the underside of the nail plate, and the nail still looks mostly normal.
As the infection progresses over weeks to months, the nail starts to thicken. You may notice the color spreading from the tip toward the base, and the texture becoming chalky or crumbly at the edges. Eventually, the nail can warp in shape, develop a foul smell, or begin separating from the nail bed entirely. That separation creates even more space for fungal growth, which is why untreated infections tend to worsen rather than resolve on their own.
Why It Spreads Once Established
Nail fungus is notoriously persistent because the nail plate itself has no blood supply. Your immune system can fight infections in living tissue, but a nail is made of dead keratin cells stacked in dense layers. Once fungi are embedded inside, immune cells simply can’t reach them. The infection sits protected inside the nail while continuing to produce spores that can spread to adjacent nails or to other people.
This is also why treatment takes so long. Oral or topical antifungal medications can kill the fungus at the nail bed and in newly growing nail, but the already-infected portion of the nail has to physically grow out and be trimmed away. For toenails, that process can take 12 to 18 months. Starting treatment early, when the infection is still a small spot near the nail tip, gives you the best chance of clearing it before it reaches the base of the nail and becomes much harder to eliminate.