Respiratory syncytial virus infection is restricted to the respiratory tract. The virus initiates infection in the epithelial cells of the upper respiratory tract. Spread of the virus down the respi-ratory tract occurs by cell-to-cell transfer of the virus along the syncytia from the upper respiratory tract to the lower respira-tory tract, resulting in pneumonia. The virus usually does not cause any viremia or systemic spread. The virus causes necrosis of the small airway epithelium, plugging of the lumens with exudates, and edema, leading to obstruction of the normal air-ways of the young infants. The host’s immune response plays a major role in the pathogenesis of bronchiolitis observed during the period of illness.
Humoral antibody plays a minimal role in the host immunity against RSV. Maternal antibodies do not protect the infant from infection. CMI appears to play an important role in recov-ery from infection. Natural infection by RSV does not prevent reinfection by the virus.