S4·E20 The Mephisto Strain

Alex struggles to cure Nick, and has to keep him alive, after he is infected by a Denomic parasite which makes him see evil all around him. The episode begins with Alex working in the control room when Nick bursts in through the front door. He chases after Alex, trying to kill her. She manages to find a torch and smashes it off his head and knocks him out. She ties him to his bed. He wakes up and threatens to kill her, constantly shouting and screeming that he is going to kill Alex and shouting that he can see evil all around him. We learn that he has been infected by a demon who causes him to hallucinate. This leads Nick to remember events from the last year (evil events), in which he was hurt or people around him were hurt. As Nick remembers the past events, Alex researches the Legacy database and she realises that Nick has the Mephisto Strain. With the hepl of the Database, Alex finda a cure, which heals Nick.