Acute bronchitis, swelling in your bronchi, is usually caused by viruses — the same viruses that give you the common cold. The viruses attack the insides of your airways and infect them. Your airways react by getting red and swollen, and by making extra mucus.
Acute bronchitis can also be caused by things that irritate your airways, such as smoke.
Po, J.Y., FitzGerald, J.M., & Carlsten, C. (2011). Respiratory disease associated with solid biomass fuel exposure in rural women and children: systematic review and meta-analysis. Retrieved July 18, 2014, from https://www.ncbi.nlm.nih.gov/pubmed/21248322