My biggest piece of advice for those is to have a good idea what you’re hoping to get out of the fellowship. Then make sure you’re asking your potential host sites questions to ensure you can do those things/build those skills at that site.
For example, I wanted more extensive analytics experience and my host site doesn’t do as much of that. I wanted more experience with SAS and was told I’d have acces to it, but it turned out my section doesn’t have a license and they are expensive. we ultimately got it, but it was a pain.
so host sites are very different it what they offer. some are data heavy, project heavy, some more outbreak heavy (if ID focused) some throw you more into routine epi work and it’s hard to get to your work done for your competencies, some are the opposite of that. so just really ask a lot of questions of your potential host sites.
also don’t assume all the action happens in the big cities. sometimes the smaller places can actually move you around more and give you exposure to more things. ie South Dakota doesn’t have a bunch of regional and local health departments doing big epi investigations. in my state though, the local health depts lead them and get some support from the state. so if you’re at the state, it’s harder to get in on the frontline investigative work.
those would be my tips.