Eighty-some-odd years ago, only buildings and bridges could be under stress, technically speaking. Back then, "stress" was strictly an engineering term that referred only to mechanical forces acting on physical structures.

In the 1920s, physiologist Walter Cannon first used the term "stress" to describe the body's response to unpleasant conditions. Cannon also identified and named the "fight or flight response." But it was Dr. Hans Selye who popularized the term "stress" and noted its deleterious effects on health.

In his book, Why Zebra's Don't Get Ulcers, Robert Sapolsky describes how Selye's bad lab technique led to his good idea. Legend has it, Selye was testing the effects of a hormone by injecting it into a group of rats. But Selye had trouble injecting the animals, and wound up traumatizing them on a daily basis, causing the rats to develop ulcers, enlarged adrenal glands and atrophied immune tissues.

How did Selye know it was the stress of the injections causing the physical changes and not the hormone under consideration? Selye's control group of rats-which received a daily injection of harmless saline-developed the same illnesses by the end of the trial.