Most of us think of stress as something that is imposed on us from the outside world. Specialists, however, define it differently: it not simply an event outside of ourselves, but our way of reacting to it. The cause of the stress, known as the stressor, may be external, such as a natural disaster or an act of war, or internal, such as feeling of anger or anxiety. It may be physical (injury or infection), economic (getting fired), or emotional (marital problems). It may be a one-time crisis or an ongoing condition.

Whatever the cause, chronic stress has a range of unpleasant effects. It can produce or contribute to muscle strain, fatigue, headaches, ulcers, asthma, back pain, digestive disorders, high blood pressure, and heart-rhythm abnormalities. What’s worse, some investigators believe that stress affects immune function, and perhaps is a factor in certain types of cancer. In a well-known study, Dr. Thomas H. Holmes and Richard H. Rahe of the University of Washington drew up a scale of stressful events that appeared to make people more susceptible to illness. The death of a spouse was found to be the most traumatic. But the severity of the reaction to most events varied from culture to culture. For instance, a jail sentence rated second for the Japanese, but only fourth for Americans.

Some researcher believe, moreover, that such everyday hassles as traffic jams, noise, or family conflict may over time, be even more harmful than sudden disasters. And one person’s trauma’s a job interview or a parachute jump may be another’s exciting challenge. In fact, while the University of Washington study found that many people fell sick after a major life change, it also found that many (about 30%) did not.