The timeline of the Spanish Flu was 1918-1919, with a first, milder wave which killed as most flus do, elderly, those with other health issues. The second wave started the fall of that same year and it was much more deadly, with a lot of cytokine storm type deaths. According to Smithsonian it eventually became less deadly: “ partly because many human immune systems now recognized it and partly because it lost the ability to easily invade the lungs. No longer a bloodthirsty murderer, it evolved into a seasonal influenza.”
https://www.smithsonianmag.com/history/journal-plague-year-180965222/
Here’s a technical paper on mutation:
“RNA viruses have much higher mutation rates, perhaps one mutation per virus genome copy. Mutations can be deleterious, neutral, or occasionally favorable. Only mutations that do not interfere with essential virus functions can persist.”
https://www.ncbi.nlm.nih.gov/books/NBK8439/