Daisy was with Jack when he jumped onto the subway tracks to save a woman who had fallen onto them. When his wife finally arrives at the hospital, she chastizes Jack for doing so because Daisy got so anxious because of the incident.
However, as the team struggles to diagnose Jack, House sees Daisy in Jack's room during school hours. He asks why she's out of school and when Eva says there is a disease at school, House realizes that although Daisy isn't sick, she carried varicella from her classmates to her father. It fooled them because he didn't develop the pustules that usually accompany the disease.