system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did the Spanish Flu end in 1919/1920?
gpt
Viruses mutate and evolve.  Over time it evolved to be less deadly.  More and more people survived catching it and developed immunity to it.  Immunity can be passed onto offspring somewhat and so children born could already have some immunity to it either preventing them from catching it or making it so they could survive it easier.

It's actually still around and people still catch it every year.