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Feeling tired is very common, especially if you have work or family problems or are under stress for other reasons. Many people say they feel tired most of the time, but very few of them really have chronic fatigue syndrome.

People with chronic fatigue syndrome (CFS for short) feel exhausted and weak a lot of the time for no obvious reason. Often they have other signs of illness, too. There is no simple test for the condition, but your doctor might say you have CFS if you:^{1 2}

• Have been feeling exhausted for no known reason for more than six months
• Also have other signs of illness, such as muscle pains, sleep problems, headaches and trouble concentrating or remembering things.

There has been a lot of research into CFS but doctors still don't know what causes it. It could have several causes.

What doctors do know now is that chronic fatigue syndrome is a real illness and not imagined, as was once thought.

Another name for the illness is chronic fatigue immune dysfunction syndrome (CFIDS for short).

Sometimes chronic fatigue syndrome starts after another illness, like a bad cold or bronchitis. And some people find it starts during a time of stress. But it can come on gradually for no obvious reason.

Some people with CFS have abnormal levels of certain hormones. Doctors have also found they have changes in their immune system (the immune system helps the body fight infection). But no one knows whether these are causes or effects of the illness.

Chronic fatigue syndrome doesn't go on for a set period of time. It can last from a few months to many years, although the symptoms might come and go during that time.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.