Exacerbations of chronic obstructive pulmonary disease (COPD) are episodes of acute deterioration in respiratory symptoms¹ that are accompanied by physiological changes² and associated with increases in airway and systemic inflammation.^3,4 These episodes are responsible for considerable morbidity and mortality, especially in patients with more severe COPD.⁵ There is consequently much interest in understanding the underlying pathophysiology of exacerbations and determining their triggers, so that appropriate interventions can be designed to prevent these events, reduce their severity and thus improve health status.

We now recognise that respiratory infections are important triggers of exacerbation. Respiratory viral infections, especially with human rhinovirus (the cause of the common cold), influenza and respiratory syncytial virus may be isolated from up to 60% of exacerbations.^6,7 Exacerbations where a virus is isolated have increased airway and systemic inflammatory changes.^3,8 The role of bacteria at exacerbation has been more difficult to determine as airway bacteria are also found in stable patients with COPD, especially those with more severe COPD. However, we now know that bacterial strain change may play a part in triggering exacerbation.⁹ Airway bacteria may be present in up to 70% of COPD exacerbations, and their isolation is accompanied by increased airway inflammatory …