Strangerville is my favorite world for many reasons but it's never one impacted my other worlds. It would have to be prompted to infect another using the strange fruit.
You can either play the story line or not. A few sims will walk around funny if you don't solve the mystery but they're already infected.
You do have to play the story line to heal them but I think it's a total of maybe 4 sims who are. When they aren't in Strangerville they act normal such as if you happen to see them at a restaurant etc. In my experience it affects nothing outside the world.