Viruses in general are propagated more efficiently in cooler drier conditions. https://en.wikipedia.org/wiki/Flu_season https://en.wikipedia.org/wiki/Flu_seaso

Viruses in general are propagated more efficiently in cooler drier conditions. https://en.wikipedia.org/wiki/Flu_season https://en.wikipedia.org/wiki/Flu_season The causes section lists many potential explanations.