I've played through Strangerville six times. I like retelling it with different spins/sub-plots. But I haven't played through it in every save.
The pack comes with a handful of sims "pre infected" - if these sims end up being pulled by the game into other worlds for festivals or to work food booths or similar calls, they will revert to their infected behavior. Likewise, as you move the story along, more npcs end up infected and will behave accordingly if the game calls for them outside of their world. I don't remember ever seeing a sim from Outside Strangerville randomly infected by the game.
If you never touch the story, then only the pre-infected sims will behave as such and (usually) only between 1am and 4 am, though some may behave infected at other times. Otherwise, the infection will say within Strangerville unless you use the item mentioned by EnkiSchmidt. (which is sometimes available from the grocer in Cottage Living.)
