ELI5 Spanish Flu - as a global pandemic, why was it nicknamed Spanish Flu?

Why was the Spanish Flu of 1918 called the Spanish Flu when it was a global pandemic? Why was the 2nd wave more deadly than the first? How many people were affected? Where did it originate?

... We explain like i'm five!

Thank you to the r/explainlikeimfive community as always and in particular the following users whose questions and comments formed the basis of this discussion: whosafossa, scriptifex, thatsnicemyman, bakanogami & alohapizzaguy

Article referenced:

https://www.smithsonianmag.com/history/journal-plague-year-180965222/

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