Chloe suffered from epilepsy and suffered a seizure while standing on a crowded subway platform. She collapsed and fell on the tracks. Only Jack did anything to try to help, but all he could do was cover her to keep her from rising up into the path of the oncoming train. Luckily, there was just enough clearance and neither of them were hit by the train.
Chloe was taken to a nearby hospital for observation, but was released the next morning.
House became convinced that Jack must have known Chloe and ordered an environmental scan of her apartment. House's hunch seemed to be confirmed when the team found a CD of Jack's band in Chloe's apartment.
However, Chloe soon ame to visit Jack in the hospital, and the team learned she received the CD from one of the nurses at her hospital. Masters realized they didn't know each other and instead collected samples for growing blood cultures