In 1982, Cynthia Toussaint, a 21-year-old North Hollywood, Calif., ballerina and actress, had a hamstring injury that wouldn't heal. Her leg pain was so severe, it was like "being doused with gasoline and lit on fire."
As her symptoms grew worse, the muscles spasms contracted her right arm and leg so tightly that her limbs "folded up."
Then came the fatigue -- so deep and persistent, Toussaint could barely lift her eyelids on awakening and often would fall out of a chair with sleepiness.
"I was bedridden for a decade and could barely crawl to go to the toilet," she told ABCNews.com. "They said it was in my head -- that I had stage fright or tendonitis from Mars. People said I was making it up."
For years, she fought with doctors, her insurance company and other skeptics, who would not believe her illness was real and painted her as lazy, complaining or just plan "nuts."
But now, scientists have identified a retrovirus that may explain the cause of CFS -- a condition once sarcastically called the "yuppie flu."
Since it was first identified 25 years ago, the syndrome often has been considered a sham, and many patients have been referred to psychiatrists when no clear diagnosis was made.
"Once my HMO even asked me to take a truth serum to see if I was lying," said Toussaint, now 48 and founder of For Grace, an organization that advocates for women in pain. "I was so ashamed."
Patients like Toussaint, who say they feel vindicated that their illness is real, are celebrating the news.
Though many in the health field await more proof, investors and medical technology companies are buzzing about the implications of finding a biomarker and possibly an eventual vaccine and treatment.
Between 1 million and 4 million Americans suffer from CFS, according to the Centers for Disease Control and Prevention. At least one-quarter of those are impaired enough to be unemployed or on disability.
This research, announced this month in the journal Science, was led by Judy Mikovits, a 22-year veteran of the National Cancer Institute and retrovirus expert at Reno, Nev.'s Whittemore Peterson Institute.
She reported 68 out of 101 CFS patients were infected with the contagious xenotropic murine leukemia virus, or XMRV. By contrast, only 3.7 percent of 218 healthy people were infected.
After the paper was published, her work showed that 98 percent of 300 patients tested positive for the retrovirus, which was found in fresh blood and plasma, as well as saliva.
XMRV also has been associated with prostate cancer, and some leukemias and lymphomas. Retroviruses carry their genetic information in RNA rather than DNA, inserting themselves into their hosts' genetic material, where they stay for life.
Although CFS only occurs in about 4 percent of the population, many more asymptomatic Americans could be carriers.
"The study is intriguing," said Dr. Joshua Prager, director of the California Center for Rehabilitation of Pain Syndromes. "There could be a sampling error, but there is hope. If, in fact, there is a marker for the disease, it is something that is truly treatable."
CFS long has baffled doctors, according to Prager.