The most common and frequent infections in humans are respiratory virus infections. Some classical respiratory viruses (e.g., rhinoviruses) enter the body through the respiratory tract, replicating and causing disease only in cells of the respiratory epithelium. Other, more systemic viruses (e.g., measles virus and severe acute respiratory syndrome [SARS] coronavirus) spread via the bloodstream and cause systemic disease; however, they also may enter through and cause disease in the respiratory tract. Although infections with systemic viruses often induce lifelong immunity against disease, respiratory viruses that do not cause viremia usually can reinfect the same host many times throughout life. Reinfection with the same virus is common because of incomplete or waning immunity after natural infection. Hundreds of different viruses cause infection of the respiratory tract, and within each virus type there can be a nearly unlimited diversity of field strains that vary antigenically, geographically, and over time (e.g., antigenically drifting influenza viruses). Specific antiviral treatment options are limited, and only a few licensed vaccines are available. For further discussion of common respiratory virus infections, see Chap. 31 and syndrome-specific chapters.
Common viral respiratory infections can be categorized in several ways, including by site of anatomic involvement, disease syndrome, or etiologic agent.
ANATOMIC SITES IN THE HUMAN RESPIRATORY TRACT
The type of respiratory disease that develops during virus infection is dictated to a large degree by the cell types and tissue organization in the respiratory tract. The vocal cords mark the transition between the upper and lower respiratory tracts. The upper respiratory tract is a complex anatomic system with interconnected structures, including the sinuses, middle-ear spaces, Eustachian tubes, conjunctiva, nasopharynx, oropharynx, and larynx. The tonsils and the adenoids are large collections of lymphoid tissue in the pharynx that participate in immunity but also are susceptible to infections. The lower respiratory tract structures include the trachea, bronchi, bronchioles, alveolar spaces, and lung tissue, including epithelial cells and blood vessels. The epithelial cell types that line the respiratory tract are varied in morphology and function, and their susceptibility to different virus infections varies. The principal types of cells in the major airways are ciliated or nonciliated epithelial cells, goblet cells, and Clara cells. Smooth-muscle cells form major tissue structures around the epithelial structures of the large airways of the lower respiratory tract down to the level of the bronchioles, and these cells are reactive to intrinsic and extrinsic signals, including viral infection or exposure to allergens or pollutants. The pathologic process of wheezing is driven by smooth-muscle contraction and obstruction of airways caused by mucous accumulation and epithelial sloughing in the lumen. Reactive airways causing wheezing are most often due to constriction of lumen size at the level of the bronchioles (which have the narrowest lumen diameter of the airways). The lung does not have smooth-muscle or ciliated cells, but instead possesses pneumocytes of types I and II. Pneumonia (Chap. 121) is an ...