The cause of Professor Yang's stroke in The Crazed by Ha Jin is actually somewhat mysterious. The first indication that this might not be an ordinary stroke comes in the opening comment that
"Everybody was surprised when Professor Yang suffered a stroke in the spring of 1989. He had always been in good health, and his colleagues used to envy his energy and productiveness."
This suggests that the stroke isn't completely ordinary. Next, when the protagonist arrives at the hospital, he gets additional details about the stroke:
"Dr. Wu had told us to keep the patient as peaceful as possible; more conversation might make him too excited. Although diagnosed as a cerebral thrombosis, his stroke seemed quite unusual, not accompanied by aphasia--he was still articulate and at times peculiarly voluble."
Since cerebral thrombosis is actually just another term for a stroke, this doesn't actually tell us much about the cause, and the lack of aphasia again suggests that this is no ordinary medical condition. The strange revelations that ensue suggest that as much as a stroke, this is a sort of psychic breakdown caused by the unbearable tensions between communist orthodoxy and literary study, and between expected party loyalty and desire for self-realization.