Fast Facts: Nail Fungus Warning Signs You Shouldn’t Ignore

If your toenail has gone unnoticed and turned yellow, thick, or crumbly, don’t disregard it. Fungal nail infection (onychomycosis) is a common problem—and catching the early warning signals can spare you months of hassle and save you from complications. Your quick, clinician-approved guide to the warning signals, who is at risk, and when to see a professional is below.

Caring for your nails goes beyond appearance — it’s part of your overall wellness. If you’d like a free guide to help you set and track your health priorities, check out the Wellness Goals Overview here.

Quick checklist: early warning signs

Color shifts: yellow, white, or brown streaks or patches. (Mayo Clinic)
Thickening and rough texture that’s hard to trim. (Mayo Clinic)
Crumbling edges or brittle, ragged nails. (Mayo Clinic)
Debris buildup under the nail (subungual hyperkeratosis). (DermNet®)
Nail lifting off the nail bed (onycholysis). (DermNet®)
Misshapen nail, foul odor, or tenderness. (Mayo Clinic)
Spreading to nearby nails — or you also notice athlete’s foot. (American Academy of Dermatology, CDC)

1. Most often missed color sign

A small pale-yellow or white spot at the tip can be the first clue. As the fungus spreads deeper, discoloration becomes more and darker. (More frequent green-black discoloration actually means bacterial infection — not fungus.) (Mayo Clinic)

2) Thickness + debris under the nail

Fungal nails get thick and unbreakable, with chalky debris accumulating underneath the plate (subungual hyperkeratosis). That’s a classic sign dermatologists look for. (DermNet®)

3) Lifting or crumbling edges

When the nail is being lifted from the bed (onycholysis), the free edge will deteriorate. These are typical of common types of onychomycosis reported in dermatology textbooks. (DermNet®)

As you learn to spot early signs of nail fungus, it’s also important to think about your broader health habits. A tool like the Wellness Goals Overview here can keep you on track with simple wellness checkpoints.

4) Pain, swelling, bleeding — or a bad odor

Fungal nails may not always be painless. Pain during walking, redness or bleeding of the nail fold, or foul odor are a concern — especially if self-care has been ineffective. These are also “see-a-clinician” red flags. (Mayo Clinic)

5) It spreads — usually from athlete’s foot

Fungus spreads from one nail to another or from athlete’s foot on the skin to the nails (and vice versa). That’s why treating both nails and skin — as well as disinfecting equipment — matters. (American Academy of Dermatology, CDC)

6) Who’s more at risk?

Risk goes up with advancing age, diabetes, immunocompromised, athlete’s foot history, wet- or tight-fitting footwear, nail trauma, and exposure to humid public areas (pool decks, showers). Athletes and psoriatic individuals also see it more often. (Mayo Clinic, DermNet®)

7) Not every thick yellow nail is fungus (testing is useful)

About half of abnormal nails are not fungal — psoriasis, injury, and other conditions can be imitators. That is why it is most commonly confirmed by a nail clipping or debris specimen (KOH/PAS, culture, or PCR) before long treatments. (AAFP)

8) When to seek a specialist

Make an appointment if you notice warning signs and one or more of the following:

You have diabetes or circulation problems.
There is pain, swelling, or bleeding around the nail.
Walking is painful or the nail is very thick/deformed.
The problem gets worse or hasn’t improved with self-care.

A board-certified podiatrist or dermatologist can make the diagnosis and chart a course of treatment. (Mayo Clinic, American Academy of Dermatology)

Smart prevention (start today)

Keep feet clean, dry, and cool; change socks; select breathable shoes.
Wear sandals when using public showers and being near pools.
Treat athlete’s foot early to avoid spread through nails.
Clean clippers/files after use; don’t share.
When getting salon services, ensure tools are sterilized. (Mayo Clinic, CDC)

The bottom line

Early on, color changes, thickening, debris, or lifting are signs that need investigation. The earlier you determine what’s happening, the easier your strategy (and the more likely your opportunity for an unblemished nail in the future). (Mayo Clinic, DermNet®)

Sources & further reading

Mayo Clinic. Nail fungus — Symptoms, causes, when to see a doctor. Updated May 15, 2024. (Mayo Clinic)
American Academy of Dermatology (AAD). Nail fungus: FAQs; Diagnosis & treatment. Updated June 26, 2025. (American Academy of Dermatology)
DermNet NZ. Fungal nail infections (overview) and Tinea unguium (clinical features). 2022–2025. (DermNet®)
American Family Physician. Onychomycosis: Current Trends in Diagnosis and Treatment. Emphasizes look-alikes and need for lab confirmation. (AAFP)
CDC. Ringworm & Fungal Nail Infections — Basics and Symptoms/Testing. 2024. (CDC)

Healthy nails reflect healthy living. To make it easier, download the free Wellness Goals Overview here and start building better daily wellness habits.