The usual cause is an infection due to a virus. The viruses responsible for acute bronchitis include those that might otherwise only cause a cold, but in bronchitis they spread further down the respiratory tree. The bronchial lining, already damaged by this infection, is then open to secondary attack by bacteria.

People who have underlying lung problems, such as chronic (ie long-standing) bronchitis, are more likely to get acute bronchitis as their lung defence mechanism against infection is lower than normal. Smoking is the commonest reason for this to happen.

Although much less common, exposure to irritant fumes and dust can be a cause of acute bronchitis.

Viruses or bacteria cause inflammation in the respiratory passages, which results in:

irritation of the respiratory passages, causing a cough.
an increased production of mucus (phlegm).
the swelling of the mucous membrane (lining) of the bronchi, along with the increased production of mucus makes the airways narrower, leading to a wheeze

The inflammation can lead to fever, fatigue, coloured spit and a general feeling of ill-health.