I'm not a biologist / epidemiologist but maybe the mutation of flu strains are synched up with this annual human behavior such that by the end of the winter most everyone has developed immunity for the current strains. By the next winter the mutations have happened again and the cycle repeats.
I'd love for this random thought to be confirmed / corrected.
for that to be true, the flu would be have to be more than than a unicellular organism in order to know what seasons are. do you have a proposal for how that would work? I'm sure there's a Nobel prize for you ($1 million dollars!) if you have something.