Bronchitis is an inflammation of the mucous membranes of the bronchi (the larger and medium sized airways that carry airflow from the trachea into the more distal parts of the lung parenchyma). Bronchitis can be divided into two categories, acute and chronic.

Acute bronchitis is characterized by the development of a cough, with or without the production of sputum (mucus that is expectorated, or coughed up from the respiratory tract). Acute bronchitis often occurs during the course of an acute viral illness such as the common cold or flu. Viruses cause about ninety percent of acute bronchitis cases, whereas bacteria account for about ten percent. Chronic bronchitis, a type of COPD, is characterized by the presence of a productive cough that lasts for three months or more per year for at least two years.

Chronic bronchitis usually develops due to recurrent injury to the airways caused by inhaled irritants. Cigarette smoking is the most common cause, followed by exposure to air pollutants such as sulfur dioxide or nitrogen dioxide, and occupational exposure to respiratory irritants. Individuals exposed to cigarette smoke, chemical lung irritants, or who are immunocompromised have an increased risk of developing bronchitis.