Acute bronchitis is typically caused by one of the viruses that infect the respiratory tract. Certain viruses, such as the influenza (flu) virus, can invade the lungs and bronchi and cause acute bronchitis. Other viruses, such as the rhinoviruses that cause the common cold, usually infect the airways higher in the respiratory tract but can cause acute bronchitis when the body's own inflammatory responses make the airways swollen, sore, and irritated. Occasionally, a bacterial infection can settle in the bronchi and cause an episode of acute bronchitis. Chronic bronchitis is caused by prolonged or chronic irritation of the lungs. Smoke, air pollution, pollen, dust, mold, and animal dander (tiny particles in the fur or feathers of animals or birds) can all act as lung irritants. Smoking is a major irritant that leads to chronic bronchitis. An infection by a virus or bacteria is also another kind of irritation that can damage the lining of the airways and contribute to chronic bronchitis.

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Acute exacerbations of chronic bronchitis or flare-ups of chronic bronchitis are often triggered by a bacterial or viral infection.

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