ABSTRACT
Influenza A and B viruses, respiratory syncytial virus (RSV) A and B, parainfluenza virus (PIV) types 1, 2, and 3, rhinoviruses, and adenoviruses target primarily the respiratory tract, and for this reason are called respiratory viruses. Although these viruses share several common features, they differ from each other in many characteristics, including their genetic material, their size, the presence or absence of an envelope, and the type of their cell receptor (Table 16.1). The upper respiratory tract, particularly the nasopharynx, is the main initial site of replication. Some, such as rhinoviruses, are mostly confined in the upper tract, whereas others, such as RSV, have a strong tendency
to initiate further replicative cycles in the lower respiratory mucosa. Respiratory viruses are the leading cause of respiratory illnesses in all age groups living in the community, and pathogenic changes resulting from a respiratory virus infection will greatly vary according to the virus type or subtype, the site of replication, the age of the subject, and the immune response. The spectrum of illnesses associated with viral respiratory tract infections ranges from a selflimited runny nose in previously healthy subjects to severe pneumonia in young infants or in immunocompromised hosts.
