Bacterial infections of the bronchial tree

In health, bacteria colonize the upper respiratory tract, but the lower respiratory tract defenses keep the lung sterile from the first bronchial division, despite being regularly challenged by bacteria from the nasopharynx and the environment. Bronchial infections reflect a failure that can be ascribed either to the virulence of the bacterium or to a deficiency of one or more of these defenses. An abnormality of the airway defenses, which may be inherited (e.g., cystic fibrosis) or acquired (e.g., following a viral infection or cigarette smoking), is present in most patients with bronchial infection, and this fact should be considered when planning investigation.