A fungal nail infection, also known as onychomycosis, occurs when fungi infect the nail or the skin beneath it. It commonly affects toenails but can also appear on fingernails. The infection develops gradually and can cause the nail to become thick, discoloured, brittle, and sometimes painful. Warm, moist environments—such as sweaty shoes or communal showers—encourage fungal growth and make infections more likely.

Early signs of a fungal nail infection include small white or yellow spots under the tip of the nail. Over time, the nail may become darker, crumbly, and distorted in shape. In some cases, the nail can separate from the nail bed or emit a slight odour. Although not usually serious, the condition can be uncomfortable and affect confidence, particularly when visible.

Fungal nail infections are caused by various types of fungi, most commonly dermatophytes, which thrive in warm, damp conditions. They can spread from person to person through direct contact or contaminated surfaces. People who frequently wear closed shoes, have sweaty feet, or use communal facilities like gyms and swimming pools are at higher risk.