Acute bronchitis is an inflammation of the lining of the bronchial tubes, the hollow air passages that connect the lungs to the windpipe (trachea). The inflammation can be caused by an infection or by other factors that irritate the airways, such as cigarette smoking, allergies and exposure to fumes from some chemicals.
Acute bronchitis caused by an infection usually starts with an upper respiratory illness, such as the common cold or flu (influenza), that spreads from your nose and throat down into the airways. Acute bronchitis does not affect the lungs like pneumonia does. Pneumonia shows up on a chest X-ray, but acute bronchitis usually does not.
Most cases of acute bronchitis are caused by viruses, although the condition also can be caused by bacteria.