Continuity mistake: While Dr. Morrow is saying, "I did some preliminary tests on David. The limbic system is definitely affected," he is holding a folder containing patient information and a picture. In the next shot the folder contains a different picture with no time to have turned any pages.
Continuity mistake: When David is on the hospital bed before his first transformation, he is wearing beige pants. During his transformation he is wearing light blue pants, but when fully transformed into the Hulk he is wearing beige 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