The U.S. Centers for Disease Control has declared chronic fatigue syndrome a legitimate disease that doctors should take seriously.

People with the illness have profound fatigue that is not improved by bed rest. The disease can persist for many years.

The department's estimates suggest as many as 400,000 Canadians may have the controversial illness.

Health Canada tried to deal with some of the stigma by recognizing the syndrome as a serious medical condition about five years ago, but the controversy continued.

The U.S. agency wanted to make a strong declaration that it is real condition even though no one knows for sure what causes chronic fatigue or how to treat it. The department has also launched a campaign to let people know it recognizes the disease.

Linda MacDonald of Edmonton has lived with chronic fatigue syndrome for 20 years and has been unable to work as a physiotherapist for the last six years. Her CFS was triggered by a common cause, a viral infection.

MacDonald said some doctors still doubt she is sick and family members tell her to push herself.

"It causes an extreme energy deficit, which I suffer from for days or weeks, meaning I can't sleep, my body shakes, it screws up my blood sugar," said MacDonald, who finds comfort in writing music.

"I wouldn't wish it on anyone, it's a curse. We have been forgotten. This is an illness that has been silenced really over the years and ignored, worldwide."

MacDonald said she's relieved the CDC has ended the silence.

"People genuinely are suffering," said Julie Gerberding, director of the influential health agency in Atlanta. "There are things we can do that will genuinely help them, and we need to take this seriously as a real illness for a lot of people."

As one of the people researching chronic fatigue, Dr. Harvey Moldofsky of Toronto said doctors are starting to see genetic differences in people with the syndrome. Correcting them may be the basis for new drugs, he said.

"Hopefully [the CDC announcement] will stimulate the interest to delve into this mystery and stir the interest of the pharmaceutical community," said Moldofsky, medical director of the sleep disorders clinic at the University of Toronto.