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."
Toussaint eventually got relief from what was diagnosed as complex regional pain syndrome, but what she believed was chronic fatigue syndrome persisted.
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.
The CDC is specific in its diagnostic criteria: Patients must exhibit unexplained fatigue that is not related to exertion and cannot be relieved by rest. At least four other symptoms also must be present for six months of more, including impaired memory, unrefreshing sleep, muscle aches, joint pain, headaches of a new kind, sore throat or tender glands.
Rivka Solomon's battle with CFS began two decades ago at the age of 21 when she and two best friends contracted infectious mononucleosis.
"They were sick for a week and I was bedridden for a year," said Solomon, now 47 and founder of the Boston-based women's empowerment group, That Takes Ovaries.
Seven years later, her symptoms intensified after a bout with "walking pneumonia," and she was forced to give up a career in international politics and to confine her work to writing her book, "That Takes Ovaries," about women being "bold, gutsy and courageous."
"It's unrelenting fatigue and comes with non-restorative sleep," she told ABCNews.com. "You are damned exhausted all the freaking time. It's a deep, to-the-bone exhaustion.
"I feel like taking a shower," she said, "but putting my arms up to wash my hair is too much work. And it doesn't ever go away."
Like Toussaint, Solomon experiences a "brain fog," which feels like "you are thinking through a thick cloud of pea soup," and chemical and perfume smells make her "dizzy and wacky."
The very name itself is a stigma, according to Solomon: "It's so pathetic, like calling Parkinson's 'shaky person syndrome.'"
The CDC has called the new CFS research exciting, but preliminary.
"If I don't know the nature of the cases and controls, I can't interpret the findings," Dr. William C. Reeves, who directs public health research on the syndrome, told The New York Times.
"We and others are looking at our own specimens and trying to confirm it," he said. "If we validate it, great. My expectation is that we will not."
The CFS debate goes back to an outbreak in Lake Tahoe, Nev., in 1984. Several hundred patients developed flu-like symptoms, fever, sore throat, headache and neurological problems like memory loss.
The CDC dismissed the epidemic at the time, even though the patients were infected with several viruses, and suggested these were psychiatric problems, according to Hillary Johnson, the author of "Osler's Web: Inside the Labyrinth of the Chronic Fatigue Syndrome Epidemic."
The name was coined in 1987 and "functioned as kind of a social punishment," Johnson said in an op-ed piece for The New York Times. CFS, which now carries the scientific name X-associated neuroimmune disease, is associated with a high suicide rate.
In 1991, Dr. Elaine DeFreitas, a virologist at the Wistar Institute in Philadelphia, found retroviral DNA in 80 percent of 30 her patients with chronic fatigue. Some of them also had rare forms of cancer.
The CDC tried to replicate her efforts, but ended research prematurely and later criticized her work. The CDC acknowledged in 1999 that it had diverted millions of dollars allocated by Congress for CFS to other programs.
But at about the same time in the 1990s, University of Miami researcher Dr. Nancy Klimas pioneered lymph node extraction therapies for what was then called chronic immune activation syndrome.
Now, drug companies are taking an interest in her work treating eight patients with reverse transcriptase inhibitors, antiretroviral drugs similar to those used today in HIV/AIDS patients.
"They went from very ill to much, much improved," said Klimas, who now directs the Gulf War Illness and Chronic Fatigue Syndrome Clinic at the Miami Veterans Affairs Medical Center.
One company that hopes to find new therapies with Klimas is Ohio-based Neo Probe, which explores activated cellular therapy technology to treat cancer, as well as viral and autoimmune diseases.
According to Frederick Cope, vice-president for pharmaceutical research and clinical development, the one "lingering question" is whether Klimas' sequencing matches those done in the Reno research.
"I wouldn't necessarily assume that it is," Cope said.
Another is getting more information on the long-term effect of this therapy, "not only that is safe, but that it is effective."
But Klimas said, "It's exciting to be excited. It's been a tough field to be in. Like Rodney Dangerfield, we don't get no respect."
The research also may bring new insight into other diseases like Gulf War illness, autism and even schizophrenia.
Klimas understands the urgency in finding vaccines and treatments.
"I take care of HIV patients all day and chronic fatigue patients another day," she said. "Between the two, the CFS ones are the more miserable. Most of my HIV patients are hale and hearty. My CFS patients cannot even participate in the care of their own families."
But no one is more excited than the CFS patients themselves.
"The news completely changed my life," said Solomon. "There's a skip in my walk because, for 20 years, even though no doctor ever discounted me, it's been a real burden and embarrassment. It means there is hope."
"[Chronic fatigue syndrome] doesn't kill you like HIV/AIDS did before treatments, but it really does limit you. You have a very small life," she said. "Now we can focus on strategies and maybe I can get my life back before I am dead."
Click here for more information on the Chronic Fatigue and Immune Dysfunction Syndrome Association of America.