When these tubes get infected, they swell, and mucus forms. Mucus is the material that comes up when you cough. The swelling of the tubes makes it more difficult for you to breathe. Acute bronchitis is almost always caused by viruses that get into the bronchial tree. The same viruses that cause colds can cause acute bronchitis.
Viruses attack the lining of the bronchial tree, causing damage.
As your body fights back against these viruses, more swelling occurs and more mucus is made. Even though your body kills off the viruses, it takes time for your body to repair the damage caused by the infection. During this time, you may continue to cough and wheeze. Anything that causes further damage to the bronchial tree, such as cigarette smoking, will lengthen the time it takes for you to get better. Acute bronchitis is spread from person to person by coughing.