HENGYANG, China – Dr. Chen Yuna had just eaten her lunch and was seated at her desk updating patients' medical records when a masked man entered her office. He pulled out a dagger and stabbed her 28 times in her neck, chest, stomach and elsewhere. Then he left her to die in a pool of blood.

He knew the hospital well enough to slip out easily: Before he became Chen's killer, the man had been her patient.

Chen's murder in central Hunan province is one of thousands of violent attacks in recent years by patients that have crystallized public discontent with China's health care system, the largest in the world.