The General Adaptation Syndrome (GAS) was developed by Han Seyle in the 1930's and established in the 1950's. He proposed that all animals including people go through the same three stages whenever they experience prolonged stress.
Our bodies have ways to deal with stress, but they also have limited resources and energy. If stress continues for too long, our bodies become exhausted. We don't always go through all three stages and in most cases the stress ends before we reach the exhaustion stage.
General Adaptation Syndrome
The body prepares to fight.
Many of the stress-related illnesses such as heart problems or ulcers correspond to the alarm stage. Hormones are released that cause various changes in the body (i.e. cortisol which reduces swelling. Norepinephrine and epinephrine cause general arousal and increases the heart rate.)
The body adapts.
Occurs when the stress continues. An animal appears "normal" again even though the physiological changes that occurred in the first stage continue but lessen. Some physiological problems diminish (e.g., if weight was lost, it is often restored.)
The body wears itself out
Eventually, the body runs out of resources. The body can no longer maintain the physiological changes, the immune system weakens, the symptoms of the first stage reappear, diseases, and even death may occur.