To grossly oversimplify, Selye discovered and documented that stress differs from other physical responses in that stress is stressful whether the news is good or bad, whether the impulse is positive or negative. He called negative stress distress and positive stress eustress.
Selye is said to have discovered the stress syndrome when in medical school he observed that people who had various illnesses seemed to share a quality of "sickness" that was highly similar.
He was the author of Stress without Distress (1974) and The Stress of Life (1956). He was a professor and director of the Institute of Experimental Medicine and Surgery at the University of Montreal.