The lungs contain thousands of tiny airways that are lined on the inner surface by a fine mucous membrane. When a person developes bronchitis the airways become inflamed and the mucous membrane swells and produces mucous, which causes coughing, wheezing and difficulty breathing. Infections with viruses and bacteria may cause bronchitis, however the mucous membrane may become inflamed as a result of exposure to allergens, chemicals or irritants. Acute bronchitis may resolve spontaneously after a few weeks, but sometimes the inflammation persists, or recurs for months. Risk factors for bronchitis include: respiratory tract infections, respiratory allergies, immune suppression, smoking and poor air quality. Symptoms include a productive or dry cough, wheezing and shortness of breath. Fever and fatigue may be present. The diagnosis is often made clinically. Confirmatory investigations may include chest X-ray, lung function test, oxygen saturation, bronchoscopy and sputum culture.