Nail fungus, also called onychomycosis, is a fungal infection that can affect the toenails or fingernails. It is more common in toenails because shoes can create a warm, moist environment where fungus can grow. Nail fungus is usually not an emergency, but it can be persistent, slow to improve, and sometimes difficult to tell apart from other nail conditions. Changes in nail color, thickness, texture, or shape should be evaluated if they are worsening, painful, spreading, or o