system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Where does the flu go when it is not "flu season" and how does it come back? And why is there a season for it at all?
gpt
The exact reasons why flu does exactly what it does have never been totally confirmed. The basic answer is that flu doesn't totally go away, it just circulates at much lower levels. Other things we do know:

*Each hemisphere has flu season in its colder months (summer in the U.S. is winter+flu season in Australia, summer in Australia is winter+flu season in the U.S.) Flu seems to spread better when people are stuck indoors in dry cold air. 

*The influenza virus has a bunch of different strains, which can evade the immune response to other strains. Each season, one strain breaks out and becomes predominant. 

*Every few decades or so, a radically new strain shows up, as flu can do some weird genetic shuffling that makes something our immune systems haven't seen before. The new strain can set off a pandemic, then later become one more of the usual cast of circulating strains.