Interference between outbreaks of infection with RSV and influenza virus was first suggested by Glezen and Denny in 1973. They observed that RSV outbreaks seemed to be interrupted by the onset of influenza virus infections. This finding was supported by Norwegian observations which showed that RSV and influenza virus outbreaks usually did not reach their epidemiological peaks during the same period. Later studies have supported these findings. It should, however, be mentioned that these observations are not unambiguous.