Have made some amendments to make better reflect this source: Wedzicha JA, Seemungal TA. COPD exacerbations: defining their cause and prevention. Lancet. 2007 Sep 1;370(9589):786-96. doi: 10.1016/S0140-6736(07)61382-8. PMID: 17765528; PMCID: PMC7134993.