Controversy also surrounds triggers of AE-COPD. Bacteria and viruses are involved in some episodes, but the relative importance of each is intertwined with disputes over the definition of AE-COPD. Progress at linking specific pathogens to molecular pathogenesis has been slow, both due to their diversity, and to the high rates of bacterial colonization of patients with COPD, even in the stable state. Moreover, in many AE-COPD cases, no pathogen can be identified. Without negating the value of analyzing infections with specific species of pathogens, it appears that progress in molecular pathogenesis could be accelerated by focusing on unifying features of the pulmonary immune response during AE-COPD.