Extensive multiplication produces a secondary viremia 5-7 days later with spread to the mucosa of the respiratory, urogenital or gastrointestinal tracts or the central nervous system. The RS virus initiates a local infection in the upper or lower respiratory tract but illness varies with age and previous experience. The virus infects ciliated epithelia of the nose, eye and mouth and remains generally confined. Virus spreads extracellularly and by fusion. Severe disease may present as bronchiolitis, pneumonia or croup, particularly in infants.
Some evidence suggests that there are possible immunopathologic mechanisms involved.
Interferon limits viral spread while IgM and IgG are protective. Long lasting IgG affords life-long immunity. Life long persistance of IgG affords protection. Breast milk may contain neutralizing factors as well. The virus is endemic in some areas and epidemic at times.