"I had a handful of good days when I didn't crash at 2 o'clock in the afternoon," says Chris Larson of Annandale, recalling how she felt last winter. "On those days I could actually get up in the morning, do some things around the house and make it through dinner." Usually, however, she was in bed by 4 p.m.

Larson, a career flight attendant, has been on medical leave since late 1985. "I can't be on my feet for more than an hour," she says. Last winter, she worked in a low-stress office job two afternoons a week and then would retreat to her bed or the living room couch while her husband, a pharmacist, did most of the household chores. Every few days, she felt energetic enough to walk the dog.

"In the beginning," Larson says, "I thought this was how you were supposed to feel at 37."

Today, Larson is feeling somewhat better. She goes to her office job three afternoons a week, recovers more quickly from being overtired and can stay awake until after 11. She attributes her improvement to an increased dose of the anti-depressant medication Elavil, but her diagnosis is not depression. Larson's mysterious, chronic fatigue, which still keeps her from the stewardess job she loves, has been ascribed to a pervasive virus, the Epstein-Barr virus, which is known to account for mononucleosis, B-cell lymphoma and certain cancers in Africa and Asia.

Her doctor calls Larson's condition chronic Epstein-Barr virus (CEBV) syndrome. Journalists have called it "the yuppie flu" after the kind of people -- young, ambitious, professional achievers, mostly in their thirties and forties and mostly female -- who tend to get the condition.

Chronic fatigue syndrome has been dismissed by some as purely psychosomatic, a charge that is difficult to refute because of the subjective nature of the symptoms. Victims feel overwhelming tiredness, muscle aches, joint pains, headaches, memory loss, dizziness, depression -- all conditions that could point to a wide range of disorders.

But despite a certain amount of skepticism and yuppie jokes, the syndrome has been receiving attention recently from prominent researchers, who see it as a window into the workings of the immune system and its intimate link to stress.

"Research is moving along rapidly," says Dr. Gary Holmes, medical epidemiologist at the Centers for Disease Control who conducted CDC's investigation of the first chronic fatigue syndrome outbreak, which occurred in Lake Tahoe, Nev., in 1985. "It's definitely a syndrome that people really experience, but I don't think we're any closer than we were six months ago to understanding just what causes it."

Although its name implies that the Epstein-Barr virus causes CEBV syndrome, Holmes says any number of factors, not all of them even viruses, could set off chronic fatigue. Last April, the CDC hosted a conference to discuss terminology, and rejected a few -- including CEBV syndrome and the term that is used in England, post-viral fatigue syndrome -- as imprecise or misleading.

The agency finally settled on the unwieldy phrase they had used since the syndrome was first identified, chronic mononucleosis-like syndrome. "We kept it because it was just about as good as any of the others," he says. "But I admit it is long."

The appeal of EBV as a cause of this syndrome is enormous. EBV, named for the two scientists who identified it in 1964, is known to cause infectious mononucleosis, the "kissing disease" (so named because it can spread through saliva) that causes a similar tiredness and achiness in large numbers of college students and other young adults.

A member of the herpes virus family, EBV lodges permanently in an individual's white blood cells, or lymphocytes, and a sudden stress can lead to its reactivation. Other herpes viruses, such as those responsible for cold sores, genital herpes, chicken pox and shingles, also remain dormant for years and can flare up during times of physical or emotional stress. Almost all American adults, some 90 percent of those over 30, show the footprints of EBV exposure in the form of antibodies to the virus's protein. The majority never experience illness at all, especially if first exposure occurs during childhood. But the EBV antibodies remain in the bloodstream for a lifetime.

Now that medical laboratories have kits available to test for EBV antibodies, doctors can look for them in patients complaining of fatigue. But because most people, sick or well, have these antibodies, it follows that most of the fatigue patients do, too.

And there's the rub. Some victims of chronic fatigue syndrome have abnormally high levels of these antibodies, but many victims have antibody levels, or titers, that are quite normal. A CDC study published in the May 1 Journal of the American Medical Association reported that the Lake Tahoe-area victims also had higher antibody titers against cytomegalovirus, herpes simplex virus types 1 and 2, and measles.

And according to Dr. James Jones, an investigator at the National Jewish Hospital in Denver, some chronic fatigue victims don't have any EBV antibodies at all -- a finding that is itself suggestive of something gone awry.

"We're looking at these patients without high titers," says Jones. "Maybe they are suffering from chronic EBV infection because they are not able to make antibodies to the virus."

The Epstein-Barr virus, according to Jones, might account for the symptoms directly, might be aggravated by the presence of other viruses, might inhibit a more generalized immune response or might be totally unrelated to certain cases of chronic fatigue.

"There may be a genetic predisposition to having problems with the EB virus," he says. "Maybe the patient can't control it properly or excretes more of the virus than others do."

Or maybe the EB virus is just a red herring. According to Dr. Stephen Straus, chief of the medical virology lab at the National Institute of Allergy and Infectious Diseases, a high EBV antibody titer doesn't necessarily indicate that the virus has been reactivated.

"An elevated titer usually reflects an abnormality in the immune system," not a reinfection with EBV, he says. Organ transplant recipients whose immune systems are deliberately suppressed to quell rejection of the organ tend to have high EBV titers, he says, as do patients with lymphoma, multiple sclerosis and other disorders of the immune system.

"Because the Epstein-Barr virus is intimately tied to the immune system -- after all, it resides in the lymphocyte -- it has the capacity to be a sentinel for what is happening elsewhere in the system," Straus says. He worries that some observers have misinterpreted a coincidence as a cause.

A malfunctioning immune system also may be suggested by James Jones' observation that the majority of his CEBV patients -- approximately 85 percent -- have allergies. This compares to just 16 percent of the population at large. Can the immune response that accounts for allergies, he wonders, somehow also account for chronic EBV infection?

The EB virus probably is at the heart of most of the cases he sees, says Jones. "The problem is in limiting our thinking to what EBV can do based on what we know about mononucleosis," he says. "Mono accounts for just one third of the illnesses associated with EBV. It's a complex virus in terms of what it can do once it infects you." While controversy remains as to how, or even whether, EBV is responsible for chronic fatigue syndrome, few researchers are questioning that chronic fatigue exists. The syndrome is easy to identify and affects most individuals in a hauntingly familiar way. "The way a patient describes the syndrome creates a picture one recognizes very easily," says Straus. "After seeing hundreds of these patients, I can tell within 20 seconds what they're talking about."

Straus says they use the same metaphors to describe the depth of their tiredness: hitting the wall, fatigue coming on in waves, always feeling like they're coming down with a cold, heaviness and achiness, lightheadedness. "I used to be a long-distance runner," says Gidget Faubian, president of the National CEBV Syndrome Association, who has had the condition for four years, "and suddenly I was always feeling as though I had just run a marathon."

Faubian's group claims 10,000 members -- three out of four of whom are women -- and 250 satellite support groups in the United States, Canada, Europe and Australia. But some federal officials think this estimate is high. "Who are you going to include in the numbers?" asks Straus. "Everybody who calls up complaining of tiredness?"

"Of the people who are told they have it," adds Holmes of the CDC, "only a fraction really do. There's been an overinterpretation of Epstein-Barr serology {blood antibodies} in diagnosing this condition." For that reason, it is important to rule out other conditions before settling on CEBV, he says.

Holmes says his agency bases the diagnosis entirely on clinical symptoms; to rely on EBV antibody levels in the blood, he says, only confuses the issue. According to Holmes, the CDC defines chronic mononucleosis-like syndrome as follows:

1. Illness for at least six months, with chronic fatigue severe enough to "significantly alter" one's daily life;

2. The presence of at least three or four of the following:fever or the subjective feeling of feverishness,chills,headaches,mental symptoms, including memory loss, confusion or a recent depression,sleep disorders, usually insomnia, muscle aches,joint pains; and

3. The elimination of other possible explanations for these symptoms, including heart disease, liver disease, lung disease, thyroid disease, multiple sclerosis, Addison's disease, lupus or cancer.

For now, doctors can offer only symptomatic treatment of the aches, pains and fevers that often accompany the syndrome. Straus has experimented with acyclovir, an antiviral drug effective against genital herpes, in treating chronic fatigue cases. But, as he reported last month to the American Society for Clinical Investigation, a study of 19 women and eight men showed that the drug was no better than an inert pill, or placebo, at relieving symptoms.

Other promising leads include collaborative work between Straus and his colleague at the National Cancer Institute, Dr. Robert Gallo, to determine the link between chronic fatigue syndrome and a cancer virus Gallo discovered last year. The new virus, HBLV (human B-lymphotrophic virus), is also a member of the herpes family.

Holmes and his co-workers are about to begin a case-control study of some 50 chronic fatigue patients in Atlanta to look for common elements in their histories -- such as exposure to a virus, chemical or other agent -- and to keep track of whether any of them recover. So far, some patients have improved after two to three years of debilitating fatigue.

Because it can look like so many other illnesses, chronic fatigue syndrome often is misdiagnosed. Because the symptoms are so subjective -- and because the victims are predominantly women -- many patients are dismissed as hypochondriacs, malingerers or neurotics.

A broadcast last summer of the ABC show "20/20" featured half a dozen patients with chronic fatigue who previously had been diagnosed with conditions that included thyroid problems, hypoglycemia, premenstrual syndrome, multiple sclerosis, nervous stomach, stress and mass hysteria. To the charge that the illness was all in her mind, one interview subject, 34-year-old Chris Guthrie, retorted, "If it was imagined, my imagination could do a lot more fun things than this."