Another Path - S2-E7
Continuity mistake: When the old Asian lady is hitting the Hulk using a wooden spoon, he is wearing brown pants. After escaping through the window, the Hulk is seen running on the street wearing blue jeans. But for the rest of his transformation he is wearing brown pants again.
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