Escape from Los Santos - S2-E11
Continuity mistake: During his second transformation, David tears his white shirt. When the Hulk is pulling Holly out of the cliff, the white shirt is nowhere to be seen. But when he takes her in his arms, he is wearing the torn white shirt.
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