Bronchitis is an inflammation of the mucous membranes of the deep inner lung passages called the bronchial tree.

Bronchitis may be either acute or chronic. Acute bronchitis is frequently caused by a viral or bacterial infection. Acute bronchitis may also result from irritation of the mucous membranes by environmental fumes, acids, solvents, or tobacco smoke. Bronchitis usually begins with a dry, nonproductive cough. After a few hours or days, the cough may become more frequent and produce mucus. A secondary bacterial infection may occur, in which the sputum (bronchial secretions) may contain pus. People whose cough and/or fever continues for more than seven days should visit a medical practitioner.

Chronic bronchitis may result from prolonged exposure to bronchial irritants. Cigarette smoking, environmental toxins, and inhaled allergens can all cause chronic irritation of the bronchi. The cells lining the bronchi produce excess mucus in response to the chronic irritation; this excess mucus production can lead to a chronic, productive cough.

Bronchitis can be particularly dangerous in the elderly and in people with compromised immune systems. These people should see a doctor if they develop a respiratory infection.