The frequency and range of bacterial pathogens in respiratory tract infections depends on a variety of variables. The aetiology is influenced by the type of respiratory tract infection (acute bronchitis, acute exacerbation of chronic bronchitis, community-acquired pneumonia, nosocomial pneumonia), geographic and seasonal factors, and by the diagnostic methods implemented to identify the pathogens. Furthermore, there exists a considerable uncertainty concerning the clinical relevance of positive bacteriological findings in respiratory secretions because they are retrieved from non-sterile or contaminated environments. To differentiate infection from contamination, both microbiological and clinical findings have to be assessed, which might be difficult in the setting of systemic or local immunosuppression.