Bronchitis is an inflammation of the bronchi (the air passages that extend from the windpipe into the lungs) which may be caused by viruses, mycoplasma, bacteria, smoking or inhalation of chemical pollutants or dust. The cells that line the bronchi have tiny hairs (cilia) that trap and eliminate pollutants. When these cells become overly irritated, they stop functioning. Consequently, the air passages become clogged by debris, and irritation increases. In response, a heavy secretion of mucous develops, which causes the characteristic cough of bronchitis.
Brief bouts of acute bronchitis may evolve from a severe cold or flu, but may also begin without having had an infection. If you have underlying asthma, bronchitis may precipitate an asthma attack. Even if you have no history of asthma, bronchitis may trigger some asthma-like symptoms, such as wheezing and shortness of breath. Smoking is frequently associated with chronic bronchitis.