Although unpredictable, influenza outbreaks are known to occur in three patterns: Pandemics every 30 to 40 years, with high excess mortality; epidemics much more frequently, with lower excess mortality; and usually mild sporadic outbreaks. The Possibility of a swine-flu pandemic this winter, resembling that of 1918–20, is the result of a unique deviation in the epidemiology of this fascinating disease.