Nail fungus typically starts as a small white, yellow, or brownish spot near the tip or edge of a nail. As the infection grows, the nail becomes thicker, discolored, and brittle, often crumbling at the edges. It’s the most common nail disorder in adults, affecting up to 20% of people over a lifetime, with the highest rates in those over 60.
Early Signs to Watch For
In its earliest stage, nail fungus is easy to dismiss. You might notice a slight yellow or white streak along one edge of the nail, or the nail may look a little cloudy in one corner. The nail bed can start to lift subtly, creating a tiny gap between the nail and the skin underneath. There may be mild thickening that makes the nail feel slightly different when you trim it.
At this point, the nail still looks mostly normal to a casual observer. Many people assume it’s just a cosmetic issue from tight shoes or minor trauma. But the fungus is already established beneath the nail surface, and without treatment it will spread.
What Advanced Nail Fungus Looks Like
As the infection progresses, the changes become hard to ignore. The nail thickens noticeably, sometimes to the point where it’s difficult to trim with standard clippers. Discoloration deepens from faint yellow to dark yellow, brown, or even greenish-black. The texture turns rough and chalky, and the nail may crumble or break apart in ragged pieces.
In advanced cases, the nail can separate almost entirely from the nail bed, a condition called onycholysis. The exposed skin underneath is often tender. The nail itself may develop a noticeable smell caused by the fungal and bacterial buildup trapped beneath it. Walking can become painful if a thickened toenail presses against the top of your shoe.
At the most severe stage, the nail structure is essentially destroyed. The nail may not regrow normally, and the infection can spread to neighboring nails. People with diabetes or weakened immune systems face the added risk of secondary bacterial infections, which can cause the nail area to turn green or black, swell, bleed, or produce pus.
Four Patterns of Nail Fungus
Not all nail fungus looks the same. The appearance depends on where the fungus enters the nail and what organism is causing it.
- Distal and lateral subungual: The most common type. The infection starts at the tip or sides of the nail. The edge lifts, turns yellow or brown, and begins to crumble. This is the classic “fungal toenail” most people picture.
- Superficial white: Flaky white patches and small pits appear on the top surface of the nail plate. The nail looks chalky or powdery rather than deeply discolored. You can sometimes scrape the white material off with a fingernail.
- Proximal subungual: The infection starts near the cuticle and works outward. A white or yellow area appears at the base of the nail, close to the half-moon shape. This pattern is less common and sometimes signals an underlying immune problem.
- Candida-related: Caused by yeast rather than the typical fungi. The skin fold around the nail becomes swollen and red, pulling away from the nail plate. White, yellow, green, or black marks appear on the nail near the cuticle and spread outward. The nail is often tender to the touch.
Toenails vs. Fingernails
Nail fungus overwhelmingly favors toenails. The warm, moist environment inside shoes and socks creates ideal conditions for fungal growth, and toes receive less blood flow than fingers, making it harder for your immune system to detect and fight the infection early. Fingernail fungus does occur, but it’s far less common and tends to progress more slowly because your hands are exposed to open air throughout the day.
If you notice changes in a single toenail, fungus is the most likely explanation. Fingernail changes, especially across multiple nails, are more likely to point toward other conditions like psoriasis.
Nail Fungus vs. Nail Psoriasis
These two conditions can look surprisingly similar, and even dermatologists sometimes need lab tests to tell them apart. But a few visual differences are reliable clues.
Psoriasis produces small pits on the nail surface, tiny divots that look like someone pressed a thumbtack lightly into the nail. Fungus doesn’t cause this pitting pattern. Psoriasis also creates reddish-brown splotches under the nail called “oil spots” or salmon patches. If you see those, it’s almost certainly not fungus.
Both conditions can cause the nail to lift from the nail bed, but psoriasis tends to produce a reddish border around the lifted area, almost like a line of color at the edge where the nail separates. Fungus creates a more ragged, crumbly separation without that distinct colored border. Another telling sign: fungus usually starts in one nail and may eventually spread to others, while psoriasis often affects several nails at once, particularly on the hands. Fungal infections can also produce a noticeable odor under the nail. Psoriasis does not.
Why Visual Diagnosis Isn’t Always Enough
Looking at a nail can give you a strong suspicion, but visual inspection alone isn’t definitive. Many conditions mimic nail fungus, including psoriasis, repeated trauma from running or tight shoes, lichen planus, and simple aging. Standard lab tests for fungus are only about 60% sensitive, meaning they miss infections roughly 40% of the time. The most accurate method is a nail clipping sent for specialized staining, which catches about 95% of true infections.
This matters because antifungal treatments take months to work and carry their own side effects. Treating a condition that isn’t actually fungal wastes time and money. If your nail changes don’t respond to over-the-counter antifungal products within a few weeks, or if the appearance is unusual, a nail biopsy can provide a definitive answer.
Signs That Signal a Complication
Straightforward nail fungus is unsightly and frustrating, but it’s rarely dangerous for otherwise healthy people. The signs that something more serious is happening include green or black discoloration (which points to a bacterial infection on top of the fungus), swelling and redness in the skin surrounding the nail, bleeding around the nail edges, and significant pain when pressure is applied. A foul smell beyond the mild mustiness of typical fungus also suggests bacterial involvement. These complications are most concerning for people with diabetes or circulation problems, where a minor nail infection can become a gateway for deeper tissue infections.