Synchronization of ILI incidence between seasons and within seasons.

Top (A): superimposition of eleven seasons of daily ILI incidence in Israel. Starting date is June 1^st. Middle (B): superimposition of ILI incidence in seven cities in the 1998–1999 season where R_e = 1.2 was low and the correlation between the different cities is relatively weak. Bottom(C): ILI incidence in the same seven cities during the 2006–2007 season where R_e = 1.6 was high and the correlation between the different cities was strong.