Outbreaks of newly emerged respiratory infections are a continuing threat. Over the last two centuries pandemics of influenza occurred at the rate of about one in every thirty years [Nguyen-Van-Tam and Hampson (2003)]. There is also a continuing threat of the emergence of new respiratory infections, as is illustrated by the emergence of SARS and its potential re-emergence.
We focus our discussion on pandemic influenza, because the threat of pandemic influenza is currently enhanced due to the global spread of avian influenza. We contrast the results with those for SARS, because they essentially represent extremes in terms of when infected individuals show symptoms relative to the start of their infectiousness.