system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 where does the flu come from every year?

Or maybe even where does it go? I have never understood how 8 months out of the year there is no flu, and then all of a sudden you need a flu shot because the flu is going around. It just doesn’t make sense??
gpt
The flu exists all year round, it just spreads more rapidly in flu season.

Flu season is partly due to the weather. The flu can spread more easily in the cold because it survives longer outside the body when it isn't hot, humid, and sunny out. That and people tend to stay indoors more in the winter where they can be exposed to more people.

Many flu strains just never go away, they just keep coming back over and over again infecting small parts of the population that isn't immune to that strain. Most flu shots actually contain multiple strains for that reason, and the annual flu shot will likely contain the COVID 19 vaccine for decades to come (once the vaccine is available) for the same reason.

Lots of new flu and cold strains originate from South East Asia. These types of viruses are naturally found in birds and pigs and jump to humans and mutate. Which then makes them infectious to humans.

These areas of the planet are notorious for high population densities of humans living close to livestock, and for massive enclosed markets with various animal cages piled one on top of the other which is why these jumps occur so often.

These viruses then spread person to person and spread all over the globe thanks to air travel and travel in general.

COVID 19 for example originated in Wuhan and spread over the world in the exact same manner. It's believed to have mutated from a bat or pangolin virus in one of these markets. That's why the US built a virology lab in Wuhan exactly because that's where a lot of flu and cold strains originate. It's about getting ahead of the game and seeing what strains are coming next.

To answer the next question the solution is get rid of the high density animal markets. China has known about this problem for decades but it's been difficult to fix because these markets have existed for centuries. But COVID 19 has hopefully convinced them that this needs to happen.