By Dr. Brian McDonough, Medical Editor

PHILADELPHIA (CBS) - Chronic fatigue syndrome is severe fatigue that is not relieved with rest and in many cases it can be associated with physical symptoms.

The CDC has criteria for chronic fatigue: the severe fatigue lasts for more than six months. There are also four of the following symptoms: impaired memory or concentration, new headaches, tender glands, muscle pain, sore throat and unrefreshing sleep.

There have been numerous studies about it but no one is quite sure what causes chronic fatigue syndrome. In fact, it is often a diagnosis made when other common causes of fatigue are ruled out.

It is estimated that more than two million Americans have chronic fatigue syndrome and many have not been diagnosed.