Short answer: we don't know. "The exact mechanism behind the seasonal nature of influenza outbreaks is unknown." https://en.wikipedia.org/wiki/Flu_season#Caus

Short answer: we don't know. "The exact mechanism behind the seasonal nature of influenza outbreaks is unknown." https://en.wikipedia.org/wiki/Flu_season#Cause https://en.wikipedia.org/wiki/Flu_season#Cause