Tracking influenza's every movement

It’s the case of the missing flu virus. When the flu isn’t making people sick, it seems to just vanish. Yet, every year, everywhere on Earth, it reappears in the appropriate season and starts its attack. So where does it go when it disappears? Does it hibernate, lying dormant in a few people and preparing for its next onslaught? Does it bounce around from the Northern hemisphere to the Southern hemisphere and back, following the seasons?