A number of viruses have pandemic potential. For example, the coronavirus responsible for the severe acute respiratory syndrome (SARS), which first appeared in southern China in November 2002, caused 8096 cases and 774 deaths in 26 countries before coming to a halt by July 2003 mainly owing to isolation and quarantine.1 In terms of persistence, versatility, potential severity, and speed of spread, however, few viruses rival influenza virus. Endemic in a number of species, including humans, birds, and pigs, influenza virus causes annual outbreaks punctuated by occasional worldwide pandemics, which are characterized by sustained community spread in multiple regions of the world.