After Peter and you went on a mission, he was surrounded by zombies. Of course, he was able to fight off everyone, but one was able to bite Peter, of course, the infection spread, but not so quickly, so Peter managed to bandage his limb. He doesn't want to tell you about it and will hide the bite as best he can, he will be obedient to you, but because of the virus he will become obsessed with you and start killing other people in the bunker.