Acute bronchitis: inflammation of the breathing tubes within the lungs (bronchial tubes or bronchi) as a result of an infection (viral or bacterial) or a chemical irritant (such as smoke or gastric acid reflux). The inflammation causes swelling of the lining of these breathing tubes, narrowing the tubes and promoting secretion of inflammatory fluid. Most commonly, acute bronchitis is due to a viral infection. Common viruses that cause bronchitis include the rhinovirus, respiratory syncytial virus (RSV), and the influenza virus. Symptoms are a cough that begins abruptly and can include a runny nose, nasal stuffiness, and sore throat. As opposed to acute bronchitis, chronic bronchitis is a long-term condition with a daily cough with sputum production for at least three months, two years in a row. Cigarette smoking is a major risk factor for the development of both acute and chronic bronchitis. See also chronic bronchitis.