Sinusitis (also known as a sinus infection) is inflammation or swelling of the mucous membranes lining the paranasal sinuses—air-filled cavities in the skull around the nose, eyes, and cheeks. It is most often caused by viral upper respiratory infections (common colds), but can also result from bacteria, fungi, allergies, nasal polyps, deviated septum, or irritants/smoke. Acute sinusitis typically lasts up to 4 weeks and presents with facial pain/pressure (especially around the forehead, eyes, or cheeks), nasal congestion or blockage, thick nasal discharge (clear, yellow, or green), reduced sense of smell, headache, postnasal drip, cough, fatigue, bad breath, and sometimes fever or tooth pain (upper teeth). Chronic sinusitis persists for 12 weeks or longer, often with milder but persistent symptoms like ongoing congestion, facial fullness, and drainage.