Infections of the airways are common. The bacteria that cause them are usually relatively nonvirulent and are often commensals of the upper respiratory tract. In health, dissemination of bacteria from the nasopharynx throughout the respiratory tract (sinuses, eustachian tubes, lower respiratory tract) occurs constantly, for example by inhalation, but airway defense, such as mucociliary clearance and local antibodies, clear the organisms efficiently. In order to understand the pathogenesis of airways infection, attention must be paid not only to the virulence properties of the pathogen involved but also to the host. For example, viral infection or cigarette smoking may impair host defenses, permitting spread of the bacteria from the nasopharynx into other areas of the respiratory tract. The pathogenesis of airways infection involves complex interactions between bacteria and the mucosal surface, local host defenses, and various aspects of the immune system. The immune response can either support local host defenses in clearing the infection or, in some circumstances, disadvantage the host by causing damage or contributing to symptoms. Investigation and therapy must take into account these complex interactions and must also pay attention to the likely pathogens and their antibiotic sensitivities.