General adaptation syndrome is Hans Selye’s description of the body’s stereotyped response to prolonged stress. It unfolds in three phases: alarm, resistance, and exhaustion.

In the alarm phase the body mobilizes—sympathetic activation, release of adrenaline and cortisol, preparation for fight or flight. If the stressor continues, the resistance phase begins: physiological coping mechanisms remain elevated, and the organism attempts to adapt while functioning at a higher cost. When the stressor persists still longer, or when resources are depleted, the exhaustion phase sets in. Immune function declines, and stress-related illness becomes more likely.

Selye’s model was based on animal studies of diverse stressors—cold, injury, infection—that produced similar bodily changes. It established the idea that stress is a nonspecific response and that chronic activation carries a cumulative cost. Modern stress research has added greater attention to psychological appraisal, individual differences, and the specific effects of different stressors, yet the three-phase outline remains a useful organizing framework.

The body can meet a challenge. When the challenge never ends, the adaptive systems themselves begin to fail.