CHARLOTTE, N.C. -- A blood test that measures the level of a special type of white blood cell can help doctors diagnose chronic fatigue syndrome, a flu-like illness some have dubbed "yuppie flu," a scientist said Sunday.

Dr. Jay Levy of the University of California at San Francisco announced the development at a national conference on the disease here. He predicted the mysterious illness will be the "disease of the '90s" as the public and the medical community become more aware of it.

The weekend conference drew 400 people, including researchers and people who have the ailment.

Chronic fatigue syndrome is characterized by exhaustion, listlessness, joint and muscle aches, and an array of other problems that persist for more than six months, and often for years.

Tens of thousands of Americans believe they have chronic fatigue syndrome, although the true prevalence is uncertain. Because it has been widely reported among well-educated women in their thirties and forties, the syndrome was dubbed "yuppie flu."

Levy said his team was unable to pinpoint a single virus as the cause of the illness. Some research has linked the syndrome to a retrovirus, one of the viruses that causes AIDS. Other studies have implicated a herpes virus or the Epstein-Barr virus, which causes mononucleosis.

"Because of the immune profile of these patients, we believe there has been an infection by some agent, though this agent may no longer be in the body," Levy said. "Or it may be hidden. It cannot as yet be identified."

In any case, he said, the immune system of chronic fatigue patients is activated for months or years, creating high levels of a particular white blood cell, called the CD8+ cytotoxic T cell. The persistence of the white blood cell causes symptoms of a viral infection, Levy said.

The CD8+ cytotoxic cells appear to be the major cell type activated in the disease -- a signature unique to chronic fatigue syndrome. The blood test checks for high levels of those cells. Levy said the level of the white blood cells along with an examination of the patient's medical history could determine whether a patient has the ailment.

"This is a way to distinguish those with fatigue alone from those with an immune reaction that causes chronic fatigue," he said.