Visible crew/equipment: In the episode when David is working in the arcade, when the two old men drive off to kill the man in the other car, the entire cast and crew, as well as the camera and camera man are reflected in the car.
Terror in Times Square - S1-E6
Visible crew/equipment: When the two old guys drive off to kill the other man near the end, the entire crew, the camera and camera man are visible on the car doors.
Visible crew/equipment: When Colleen is driving cab you can see somebody's fingers on the seat behind her.
Answer: An illness, although it's never stated what kind of illness. David blames his dad for her death because his father never took her to the hospital. So in all likelihood, her illness was never diagnosed anyways and David might not even have known what it was.
Bishop73