Continuity mistake: When the lab is set on fire and the Hulk jumps into the room to save Elana, the Hulk that jumps through is actually a stunt double. Noted because the Hulk that jumps in isn't that tall.
Continuity mistake: When David gets out of his car and looks at the flat tire, the hubcap is gone. When he goes to the front and jacks the car up, he uses the jack handle to remove the hubcap that wasn't there before.
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