What causes nail fungus?

Fungi thrive in warm, damp spaces and can pass through tiny breaks in the skin or nail. [Athlete's Foot](/conditions/athletes-foot/) may spread from the skin between the toes to nearby toenails. Tight shoes, sweaty feet, repeated water exposure, shared changing-room surfaces, and nail trauma can create conditions that help infection persist. The nail may become yellow, white, or brown, thicker, brittle, crumbly, ridged, or partly separated from its bed. These features can also occur with psoriasis, injury, or other nail disorders, so colour and thickness alone do not confirm fungus.

Keep feet dry, change damp socks, let shoes air, and wear protective footwear in communal wet areas. Treat athlete's foot promptly and avoid sharing towels, socks, shoes, or nail tools. Trim nails carefully without digging into the sides, and disinfect or avoid tools used on an affected nail. A clinician may examine the nail and test a clipping before choosing topical or oral treatment. See [Nail Fungus](/conditions/nail-fungus/) for condition-focused guidance.

Arrange an assessment when a nail becomes thick, crumbly, discoloured, painful, or separated, particularly if the change persists despite foot care. Seek earlier advice for redness, warmth, swelling, drainage, fever, or a wound. People with diabetes, poor circulation, nerve problems, or reduced immunity should have foot and nail changes reviewed promptly rather than trimming deeply or treating blindly.

Could this be fungus or another nail condition? Should the nail be tested before treatment? Do I also have athlete's foot? Which treatment suits my health and medicines? How should I protect other nails and household members?