Infectious agents are a major cause of acute exacerbations of chronic bronchitis (AECB) and COPD. Several respiratory viruses are associated with 30% of exacerbations, with or without a superimposed bacterial infection. Atypical bacteria, mostly Chlamydia pneumoniae, have been implicated in < 10% of AECB. The role of bacterial pathogens when isolated from the respiratory tract during AECB has become better defined by application of several newer investigative techniques. Bacterial pathogens can be isolated in significant concentrations from distal airways in 50% of AECB. Specific immune responses to surface exposed antigens of the infecting pathogen have been shown to develop after an exacerbation. Emerging evidence from molecular epidemiology and measurement of airway inflammation further support the role of bacteria in AECB. When properly defined, 80% of AECB are likely to be infectious in origin.