Acute lower respiratory tract infection.

Copyright © 2008 Massachusetts Medical Society. A cute lower respiratory tract infections are a persistent and pervasive public health problem. They cause a greater burden of disease worldwide than human immunodeficiency virus infection, malaria, cancer, or heart attacks.1 In the United States, they cause more disease and death than any other infection, and there has been little change in mortality due to respiratory tract infection for more than five decades.1,2 The outcome of an acute lower respiratory tract infection depends on the virulence of the organism and the inflammatory response in the lung. When small numbers of low-virulence microbes are deposited in the lungs, an effective defense can be mounted by resident innate immune defenses, such as the mucociliary escalator, antimicrobial proteins in airway surface liquid, and alveolar macrophages. In contrast, numerous or more virulent microbes elicit an inflammatory response. Although this response serves to reinforce innate immunity and is essential to rid the lungs of microbes, it contributes directly to lung injury and abnormal pulmonary function. This article reviews our current understanding of inflammatory responses in infected lungs, emphasizing recent advances and gaps in knowledge. Much of the information originates from animal experiments; studies with human volunteers and patient-derived data are included when appropriate and available.