Toenail Fungus: Causes, Symptoms, and What to Do Next

What Causes Toenail Fungus?

Toenail fungus — medically termed onychomycosis or tinea unguium — is caused primarily by a group of fungi called dermatophytes. The most common culprit is Trichophyton rubrum, though T. mentagrophytes and other species are also involved. In some cases, yeasts (Candida species) and non-dermatophyte moulds can cause similar infections, particularly in immunocompromised individuals.

These fungi are opportunistic — they don't infect healthy, intact nails easily. They gain entry through:

• Tiny cracks or cuts in the skin around the nail
• Separation between the nail plate and nail bed (onycholysis)
• Nail trauma (even minor repeated microtrauma from ill-fitting shoes)
• Prolonged exposure to moisture that softens the nail and surrounding skin

Once established in the nail bed, the fungi feed on the keratin protein that makes up both the nail and skin. The infection is largely shielded from the immune system by the nail plate above it — which is why it persists for so long and why surface-only treatments rarely succeed.

Why Are Adults Over 40 More Vulnerable?

Age is one of the strongest risk factors for onychomycosis. Studies consistently show that prevalence increases with age — affecting an estimated 20% of adults over 60 compared to around 3% of those under 18. Several biological changes contribute to this:

Reduced Peripheral Circulation

As we age, blood flow to the extremities decreases. The nail bed and surrounding tissue receive less oxygen and fewer immune cells. This makes it harder for the body to mount an effective local defence against fungal invasion.

Slower Nail Growth

Nail growth slows with age — sometimes significantly. Slower nail turnover means infected nail tissue remains in place longer before being replaced, giving the fungus more time to entrench and spread horizontally.

Thicker, More Porous Nails

Toenails naturally thicken with age due to changes in the nail matrix. Thicker nails are harder to trim and maintain, and the increased porosity of aged nail keratin may allow fungal penetration more readily.

Cumulative Exposure

The longer you live, the more time you have spent in shared environments — pools, gyms, communal showers — where fungal spores are present. People over 40 have simply had more exposure opportunities than younger adults.

Underlying Health Conditions

Conditions common in older adults — diabetes, peripheral vascular disease, immune disorders — all increase susceptibility. Diabetes in particular is a major risk factor because it impairs both circulation and immune function simultaneously.

Recognising the Symptoms: Stage by Stage

Onychomycosis typically develops gradually. Most people don't notice it until the infection is already moderately established. Knowing what to look for at each stage helps you seek treatment earlier, when it is most effective.

Early Stage

• A small white or yellow spot near the tip of the nail (often at the corner)
• Mild discolouration that doesn't respond to cleaning
• Slight roughening of the nail surface
• No pain or discomfort

At this stage, topical treatments have the best chance of success because there is less infected tissue to address. Unfortunately, early-stage nail fungus is often dismissed as minor cosmetic blemishing and goes untreated.

Moderate Stage

• Discolouration spreading inward toward the nail base — yellow, brown, or white
• Nail thickening and surface texture changes (ridging, pitting)
• Slight brittleness or crumbling at the nail edges
• Possible early separation of the nail from the bed at the tip
• Mild unpleasant odour may develop

This is when most people begin to seek treatment. Topical treatments remain appropriate but require more patience, and consistency of application becomes critically important.

Advanced Stage

• Severe discolouration (dark yellow-brown, grey, or almost black in some cases)
• Significant nail thickening — nail may be difficult to trim with standard clippers
• Substantial nail separation from the nail bed
• Crumbling, fragile nail structure
• Possible debris accumulation beneath the nail
• Discomfort when wearing footwear
• Risk of spreading to other nails or skin

Advanced cases typically require professional assessment. Prescription oral antifungals or in-clinic procedures may be needed alongside topical treatment.

Common Risk Factors: Are You More Susceptible?

Beyond age, the following factors significantly increase your likelihood of developing toenail fungus:

• Regular use of shared facilities: Swimming pools, gym changing rooms, communal showers, and hotel bathrooms are high-risk environments for fungal contact
• History of athlete's foot: Athlete's foot (tinea pedis) is caused by the same dermatophytes and frequently precedes or accompanies nail fungus
• Narrow or tight footwear: Creates microtrauma to toenails and a warm, moist microenvironment
• Hyperhidrosis (excessive sweating): Persistently damp feet are an ideal fungal environment
• Nail psoriasis: Creates nail irregularities that can be colonised by fungi
• Weakened immune system: Due to conditions or medications
• Chronic skin conditions: Including eczema that affects the feet and nail folds

How Is Nail Fungus Diagnosed?

Visual diagnosis alone is not always reliable — several conditions mimic onychomycosis. Definitive diagnosis involves laboratory analysis of nail clippings. Options include:

• KOH (potassium hydroxide) microscopy: Quick, in-office test that can identify fungal elements
• Fungal culture: Slower (2–6 weeks) but identifies the specific organism and guides treatment choices
• PCR testing: Faster and more sensitive, increasingly used in specialist settings

If you are considering prescription antifungals, a confirmed diagnosis is important — not all nail changes respond to antifungals, and oral medications carry risks that warrant confirmed indication.

Can Toenail Fungus Spread to Other Parts of the Body?

Yes. Tinea pedis (athlete's foot) and tinea unguium (nail fungus) are closely related and frequently coexist. From toenails, fungus can spread to:

• Adjacent toenails
• The surrounding skin (causing athlete's foot or moccasin-type infections)
• Groin area (tinea cruris) if the towel used on the feet is then used elsewhere
• Fingernails (if you touch infected toenails and then touch your fingernails)

Treating nail fungus and accompanying skin infections simultaneously, and practising good hygiene, prevents ongoing self-reinfection.

For more information on treatment strategies, visit our comprehensive treatment guide or read about the stages of nail fungus progression in detail.