Some believe lower natural production of immune-system-important Vitamin D (through less sun exposure) is a factor, though studies have been mixed: http://en.w

Some believe lower natural production of immune-system-important Vitamin D (through less sun exposure) is a factor, though studies have been mixed: http://en.wikipedia.org/wiki/Vitamin_D_and_influenza http://en.wikipedia.org/wiki/Vitamin_D_and_influenza (I'd guess higher levels of sunlight might also help sterilize viruses on surfaces and suspended in the air, but have no handy references supporting that theory.) There's also the unlikely-but-fun fringe theory that fresh flu viruses regularly fall to Earth from space: http://www.panspermia.org/panfluenza.htm http://www.panspermia.org/panfluenza.htm Perhaps either a certain part of Earth's orbit, or colder hemispheric temperatures, mean more falling space flu? :)