Infections are frequent and important causes of exacerbations in patients with COPD. This article reviews underlying mechanisms and therapeutic consequences. A complex interaction exists between COPD, co-morbidities, physical inactivity and systemic inflammation. The components of the postulated chronic inflammatory systemic syndrome need to be identified in more detail; physical inactivity seems to be the least common denominator. The patient's adaptive and innate immune systems play a role for the pathogenesis of infections. When interpreting positive bacterial cultures, it is important to differentiate between colonisation and infection. The impact of viral infections in COPD exacerbation needs further clarification, including the task to distinguish acute infection from viral persistence. Community acquired pneumonias pose a special risk for patients with COPD. Clinical scores and procalcitonin serum concentrations can support decisions on whether or not to start antibiotic treatment. Antibiotics probably do not need to be taken for longer than 5 days, since their efficacy does not increase after longer treatment, while adverse events rise in frequency. Hospitalisations for respiratory exacerbations are associated with increased mortality in COPD.