The lung is exposed to enormous quantities of air and to potentially infectious agents, but serious infections rarely occur, a testament to the extraordinary natural defences of the respiratory tract. The most common causes of bacterial lung infections in normal hosts include Streptococcus pneumoniae, Haemophilus species, Staphylococcus aureus and Mycobacterium tuberculosis. In compromised hosts, the bacterial causes of pneumonia are much broader, including species not usually considered of high virulence in humans. Indeed infection with one of these unusual bacterial species demands a search for an immunocompromising condition. Normal defences of the respiratory tract include non-specific physical factors (the 'mucociliary escalator'), and innate factors, including defensins, lysozyme and phagocytic cells (polymorphonuclear leukocytes, pulmonary alveolar macrophages and dendritic cells). Antibacterial defences are enhanced by opsonins, including those intrinsically present (surfactant and complement components) and induced immunoglobulins. Immunocompromising conditions, in which bacterial lung infections frequently occur, include (but are not limited to) hypogammaglobulinaemia, chronic granulomatous disease and primary ciliary dyskinesia. Each of these conditions illustrates the essential role of the disabled element of the innate and adaptive immune system in maintaining sterility of the lower respiratory tract.