When these tubes get infected, they swell, and mucus forms. Mucus is the material that comes up when one coughs. The swelling of the tubes makes it more difficult for one to breathe. Causes 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 the body fights back against these viruses, more swelling occurs and more mucus is made. Even though the body kills off the viruses, it takes time for the body to repair the damage caused by the infection. During this time, one 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 one to get better. Acute bronchitis is spread from person to person by coughing.