The Pandemic Legacy rules explicitly say you get to choose. And explicitly say when in the setup procedure you make that choice.
If you've overlooked that rule, it may be worth re-reading the entire rulebook to make sure there's nothing else you've missed. Everything matters!
this is excellent advice. my group definitely missed a few things. i would also recommend re-reading the rules before each subsequent game as it's very easy to forget new rules that were added one game ago but outside of that largely haven't been rules before.