Bronchitis is caused by inflammation of the bronchial tubes, which are the main airways that carry air to your lungs. The causes vary depending on whether the bronchitis is acute or chronic:

• Acute bronchitis is most commonly caused by viral infections, including the same viruses that cause colds or the flu. Bacterial infections can sometimes cause it as well. When infected, the bronchial tubes swell and produce excess mucus, which narrows the airways and makes breathing difficult.
• Chronic bronchitis is usually caused by long-term irritation of the bronchial tubes from smoking or exposure to air pollution, chemical fumes, dust, or secondhand smoke. This leads to ongoing inflammation and mucus production.

Risk factors for bronchitis include smoking, exposure to lung irritants (such as pollution, dust, and fumes), having asthma or allergies, a weak immune system, and conditions like gastroesophageal reflux disease (GERD). While the bronchial inflammation itself (bronchitis) is not contagious, the viruses or bacteria that cause the infection can be spread to others. In summary, bronchitis is caused by viral or bacterial infections (acute bronchitis) or by prolonged exposure to irritants like tobacco smoke and pollution (chronic bronchitis), leading to inflammation and mucus buildup in the airways. This information is supported by medical sources including WebMD, the American Lung Association, Mayo Clinic, and others.