Nail fungus, medically known as onychomycosis, is a common fungal infection that affects the fingernails or toenails, most often the toenails. It occurs when fungi invade the nail through small cracks or separations between the nail and nail bed, leading to discoloration, thickening, brittleness, and distortion of the nail. The affected nail may appear yellow, white, or brown and can become crumbly or separated from the nail bed over time. Risk factors include warm, moist environments, aging, poor circulation, diabetes, and frequent use of communal showers or locker rooms. Although usually not painful in early stages, untreated nail fungus can spread, worsen, and cause discomfort or secondary infections.