Played Sims who get infected will wake up at 1 am in zombie mode with needs filled and will have minimal command you can give them, mostly involving interacting with the weird plants. It wears off around 4 am. Also, if an infected sim is in the vicinity of a scanner (which you can only get by following the plot line) they will go into zombie mode and attack the person with the scanner and continue as they do at night for a few hours afterwards. Your active household members can only get infected through your actions, such as eating the fruit or a vampire drinking from an infected sim.
Yes, without mods the infected premade sims will be like that forever if you don't solve the mystery. The unplaywd local infected Sims show up in zombie mode at more random times of day rather than just at night or when triggered by a scanner.
