Nail Fungus · Guide
Toenail Fungus: Symptoms, Causes and Stages
By PLACEHOLDER: Your Name · Updated July 2026
That yellow, thickened toenail that keeps getting worse is usually the first sign of a fungal nail infection. Toenail fungus is common, slow-moving and easy to miss until the nail is visibly damaged. Here is exactly what the symptoms look like, what causes it, how it spreads through its stages, and the other conditions that can be mistaken for it.
Key takeaways
- Toenail fungus (onychomycosis) is a fungal infection of the nail, most often caused by dermatophytes, the same fungi behind athlete's foot.
- Classic signs: thickening, yellow or white discoloration, brittle crumbly edges, debris under the nail and the nail lifting away from the nail bed.
- It usually starts as a small white or yellow spot under the tip of the nail, then spreads deeper and across the nail over months.
- Not everything that looks like fungus is fungus. Psoriasis, injury and other conditions can mimic it, so a doctor may take a sample to confirm.
What is toenail fungus?
Toenail fungus, known medically as onychomycosis, is an infection of the nail by microscopic fungi. Most cases are caused by dermatophytes, a group of fungi that feed on keratin, the protein that makes up your nails, hair and the outer layer of skin. Less often, yeasts (such as Candida) or molds are responsible.
Toenails are affected far more often than fingernails. Toes spend the day enclosed in warm, dark, sweaty shoes, which is close to a perfect environment for fungus, and blood flow to the feet is slower, which makes it harder for the body to clear an infection on its own.
For a full overview of the topic and where to go next, start at our nail fungus hub.
Toenail fungus symptoms and signs
Early on, a fungal nail can be easy to overlook. As the infection develops, the most common signs are:
- Thickening of the nail, so it feels harder to trim.
- Discoloration: yellow, white, brown or occasionally greenish patches or streaks.
- Brittle, crumbly or ragged edges that flake or break off.
- Distorted shape, with the nail curling, lifting or growing unevenly.
- Debris under the nail, a buildup of chalky or dusty material beneath the nail plate.
- Separation from the nail bed (a condition called onycholysis), where the nail lifts away and looks detached.
- A slight odor in some cases.
Fungal nails are usually not painful in the early stages. Discomfort, pressure or pain tends to appear later, once the nail is thick and distorted enough to press against footwear. If your main goal is clearing an existing infection, see our guide on how to get rid of toenail fungus.
Fungus or just a stain?
Cosmetic staining from dark nail polish, tobacco or certain foods sits on the surface and buffs off. True fungal discoloration is inside and under the nail, comes with thickening or crumbling, and does not scrub away.
The stages: how toenail fungus progresses
Fungal nail infections tend to move through a predictable progression, though the speed varies a lot from person to person.
1. Early spot
The infection often begins as a small white or yellow spot under the tip or side edge of the nail. At this stage there is little thickening and it is easy to dismiss.
2. Spreading
As the fungus feeds on keratin, the discoloration spreads deeper into the nail and back toward the cuticle. More of the nail turns yellow, white or brown.
3. Thickening and crumbling
The nail becomes noticeably thick, brittle and crumbly. Debris builds up underneath, the shape distorts, and the nail may start to lift from the nail bed.
4. Advanced
In advanced cases the nail can become severely misshapen, crumble at the edges, and cause discomfort. The fungus can spread to other toenails or to the surrounding skin as athlete's foot, and can be passed to other people through shared surfaces.
Because fungus spreads, mild cases are worth addressing before they reach the later stages. Some people start with gentle at-home approaches; see our overview of home remedies for nail fungus.
What causes toenail fungus?
The fungi that cause onychomycosis are extremely common in the environment. An infection usually takes hold when those fungi meet the right conditions and an entry point into the nail. The main causes and risk factors are:
- Warm, moist environments: sweaty feet and enclosed, occlusive shoes create ideal growing conditions.
- Communal wet areas: locker rooms, gym showers, swimming pools and shared bathrooms, where fungi live on wet floors and spread barefoot.
- Athlete's foot: an untreated fungal skin infection can spread from the skin to the nail.
- Nail trauma: stubbing, repeated pressure from tight shoes, or a cracked nail gives fungus a way in.
- Older age: nails grow more slowly and circulation declines with age, both of which raise the risk.
- Diabetes and poor circulation: reduced blood flow makes infections harder to fight off.
- A weakened immune system, whether from illness or medication.
- Family history: a genetic tendency toward fungal nail infections runs in some families.
How toenail fungus spreads
Fungal nail infections are contagious. The fungi and their spores spread through:
- Direct contact with an infected nail or skin, including your own other toes.
- Contaminated surfaces, such as damp floors, bath mats and shower stalls.
- Shared items, including towels, socks, shoes and nail clippers or files.
This is why the infection often appears in more than one nail over time, and why treating any accompanying athlete's foot matters. Keeping feet dry, wearing sandals in communal showers and not sharing nail tools all reduce the spread.
Conditions that look like toenail fungus
Here is an honest and important point: not every ugly nail is fungal. Studies suggest a meaningful share of nails treated as fungus turn out to be something else. Common look-alikes include:
| Condition | How it can mimic fungus |
|---|---|
| Nail psoriasis | Causes pitting, thickening, discoloration and separation from the nail bed. |
| Trauma / bruising | A blow to the toe causes bleeding under the nail (a black or dark nail) that looks like discoloration. |
| Lichen planus | An inflammatory condition that can thin, ridge and damage nails. |
| Other nail disorders | Age-related changes, eczema and certain infections can all discolor or distort nails. |
Because these conditions need different treatment, a doctor may take a nail clipping or scraping and send it for testing before starting antifungal treatment. Treating psoriasis with an antifungal, for example, will not help. If you want to compare the proven options once fungus is confirmed, see our roundup of Best Nail Fungus Treatments 2026.
When to see a doctor
Diabetes and circulation: do not wait
If you have diabetes, poor circulation or a weakened immune system, treat any nail or foot change as a reason to see a doctor promptly, even a minor one. In these groups a small fungal or foot problem can escalate into a serious infection. Seek care quickly for pain, spreading redness or swelling, or pus. This article is general information, not medical advice.
For most other people, it is worth seeing a doctor or podiatrist if:
- The nail is painful, spreading or bothering you cosmetically.
- Home measures have not helped after a reasonable trial.
- You are not sure the problem is fungus rather than a look-alike condition.
Getting an accurate diagnosis first saves months of treating the wrong thing.
Frequently Asked Questions
What are the first signs of toenail fungus?
Usually a small white or yellow spot under the tip or side of the nail. Over time the nail thickens, discolors, becomes brittle or crumbly, and may lift away from the nail bed.
What causes toenail fungus?
It is caused by fungi, most often dermatophytes, that infect the nail. Warm sweaty feet, enclosed shoes, communal showers and pools, athlete's foot, nail injury, older age, diabetes and a weakened immune system all raise the risk.
Is toenail fungus contagious?
Yes. It can spread to your other nails and skin and to other people through direct contact, damp floors, and shared towels, socks, shoes or nail clippers.
Can something other than fungus cause a discolored, thick nail?
Yes. Nail psoriasis, an old injury or bruise, lichen planus and other conditions can look just like fungus. That is why a doctor may test a nail sample before treating, so you do not treat the wrong problem.