Everyone has stress. Whether it's the child who's anxious about a test or the adult who's struggling to get along with the in-laws, stress is a normal part of life. It's also not necessarily a bad thing. In small doses, stress can enhance our performance, help us persevere through an emergency, and push us toward higher goals. More important, stress can be essential to our survival. Here's how it works.

Imagine walking alone through a dark park late at night. Suddenly, from behind, you hear footsteps closing in. All senses go into high alert. Your pace quickens, your breathing becomes fast, and your heart begins to race as your body braces itself to defend against the potential attacker.

Meanwhile, several events are taking place inside your body so that you can be prepared to deal with what you sense might be danger. In the brain, the hypothalamus releases corticotropin-releasing hormone (CRH). CRH then triggers the release of norephinephrine, epinephrine — more popularly known as adrenaline — and cortisol, three hormones that work together to help the body brace itself to fight or flee by temporarily improving strength and agility, bolstering concentration and reaction time, and mobilizing reserves of fat and carbohydrates for immediate energy. If the person behind you grabs for your purse, you're braced to fight or to chase him down the street.

Now assume the person walking behind you passes by, and you realize he's simply running to catch a bus. The threat disappears. Your heart rate slows, and you feel calmer. Most of the hormones' levels drop. The exception is cortisol, which continues acting on the brain to halt the production of CRH in order to stop the stress response.

Under these kinds of situations, stress is a necessary defense mechanism that ensures your survival. The problem occurs when stress is chronic, as it often is when you have a chronic illness like fibromyalgia that is tiring, painful, and unpredictable. Other sources might be nightly arguments in a fragile marriage, the threat of job cuts, or even the responsibility of planning a wedding.

Living with chronic stress is harmful to your health. It keeps your body in a perpetual state of fight or flight, even when there is nothing to run from and your body isn't moving. As a result, your muscles remain tense, your mind unsettled, and your stomach uneasy. Allowed to linger, chronic stress weakens the immune system, and makes you vulnerable to everything from the common cold to anxiety and depression.