Final answer:
Acute bronchitis is most commonly caused by viruses following a cold or flu, and these include influenza, parainfluenza, respiratory syncytial virus, adenovirus, and coronaviruses like SARS. It is characterized by symptoms such as coughing up phlegm and wheezing.
Step-by-step explanation:
Viruses are the most common cause of acute bronchitis, an inflammation of the bronchi, which are the air passages that conduct air into the lungs. Viruses associated with acute bronchitis often follow a cold or the flu and include influenza virus types A and B, parainfluenza virus, respiratory syncytial virus (RSV), adenovirus, and coronaviruses, including the SARS coronavirus. These viruses lead to symptoms such as a cough producing phlegm, shortness of breath, and wheezing. Most of these respiratory infections are mild and self-limiting; they usually resolve with proper rest and care, without the need for antibiotics, which are ineffective against viral infections.