<prompt> Commonest etiological factor in acute exacerbations of COPD isViral infectionsBacterial infectionsPaiculate air pollutantsIdiopathic  <response>: Exacerbations are a prominent feature of the natural history of COPD. Exacerbations are episodes of increased dyspnea and cough and change in the amount and character of sputum.Studies suggest that acquiring a new strain of bacteria is associated with increased near-term risk of exacerbation and that bacterial infectionsuperinfection is involved in over 50 of exacerbations. Viral respiratory infections are present in approximately one-third of COPD exacerbations.In a significant minority of instances 20-35, no specific precipitant can be identifiedRef: Harrison 19e pg: 1707 