WASHINGTON— MEDICAL experts are struggling, with only limited success, to understand a mysterious illness that leaves its victims exhausted for months or years at a time.

The ailment, known as chronic Epstein-Barr virus infection, chronic mononucleosis or chronic fatigue syndrome, has stirred rising concern in public and medical circles over the last two years.

But whether patients have fallen into the grip of a new, worsening scourge or have merely succumbed to the latest health hysteria is confounding many medical researchers. And the more experts study patients who have the fatiguing ailment, the less important a culprit the Epstein-Barr virus seems.

The illness has already caused one highly publicized but sharply disputed ''epidemic'' at a resort community at Lake Tahoe, Nev., from 1984 to 1986. It may have caused an outbreak of 100 cases in the small Nevada town of Yerington from late 1985 to early 1986, according to Dr. Anthony L. Komaroff, director of general medicine at the Brigham and Women's Hospital in Boston.

''Whatever it is, it seems to be growing in frequency,'' said Dr. Komaroff, whose group has studied more than 500 patients suffering from the syndrome. ''Literally every time I say to a friend that I'm studying this illness, and then describe it, they say 'Oh, my God. My niece has it, or my next-door neighbor, or my boss.' ''

Although the syndrome has apparently never been fatal, it can unquestionably be devastating to many of those affected. Virtually all suffer persistent fatigue, in some cases so overwhelming that it curtails work, restricts social life or even becomes totally disabling. Most victims also experience low-grade fever, muscle pains, depression, headaches, recurrent sore throats, swollen lymph glands and allergies.

And many victims are unable to think clearly or to concentrate. Some have likened their condition to feeling like a Raggedy Ann doll with the stuffing knocked out or to an endless bout of flu with the added mental confusion of Alzheimer's disease.

But whether chronic fatigue itself is reaching epidemic proportions or is simply receiving an epidemic of attention remains uncertain.

The syndrome may have gained undue notoriety for a number of reasons: scientific speculations, now discounted, that the Epstein-Barr virus might be causing much of the illness; aggressive marketing of Epstein-Barr laboratory tests that are probably incapable of identifying most victims of the illness; vigorous campaigning by Epstein-Barr patient groups, and far-reaching television and magazine coverage in the wake of the Lake Tahoe ''epidemic.''

Some experts suspect that ''chronic Epstein-Barr virus syndrome'' has become the latest fad diagnosis, replacing iron-poor blood, low blood sugar and thyroid problems as a catch-all explanation for symptoms that cannot otherwise be explained.

''I don't think it's clear that there's an epidemic of this,'' said Dr. George Miller, a professor of pediatric infectious diseases, epidemiology and molecular biochemistry at the Yale University medical school. ''I would say there might be an epidemic of diagnosis.''

Dr. Gary Holmes, an epidemiologist at the Federal Centers for Disease Control who studied the Lake Tahoe outbreak, said, ''A lot more is being made of this by the lay press than it probably deserves,'' adding, ''The problem is that almost every person in the United States has symptoms that are compatible with symptoms of the syndrome.'' No Consensus on Definition

At this point, in most cases of chronic fatigue syndrome, there is no identifiable cause, cure or preventive. Nor is there even an agreed-upon definition. But there is a growing opinion that the syndrome will turn out to include a wide range of illnesses, with causes and treatments differing from patient to patient.

The evolving evidence suggests that some victims of the syndrome have a real organic illness that is not a figment of their imaginations but that it was probably premature for doctors and patients to latch onto the Epstein-Barr virus as the likely cause in most cases of the syndrome, except for perhaps a subgroup of severely ill patients.

Depending on the illness, experts speculate, the cause might be another virus or multiple viruses, other infectious agents, defects in the body's response to an infection, toxic agents, stress or other psychological causes, or perhaps a combination of physical and psychological factors interacting with one another.

To unravel some of the mysteries, Congress has ordered the Centers for Disease Control and the National Institutes of Health to study the phenomenon, and the C.D.C. has been drawing up a strict definition of chronic fatigue to make epidemiological studies more feasible. And investigators at a small number of medical centers are trying to develop diagnostic tests, treatments and vaccines. Psychosomatic or Real Disease

Some virus experts continue to believe that much of the chronic fatigue is psychosomatic. ''A lot of illness that is now being associated with chronic Epstein-Barr virus infection is probably ordinary neuroses which are manifested nowadays as tiredness,'' said Dr. Eliot Kieff, head of infectious diseases at the Brigham and Women's Hospital.

''It's a disease mostly of younger adults who are having difficulties in what are ordinarily difficult phases of life,'' he added. ''These people are very unhappy, and it's often very difficult to sort out how much of their psychological problems come from their illness and how much is the cause of their illness. Most of them do not want to see a psychologist or a psychiatrist. They're looking for a physical cause of their illness and a relationship with their physician.''

Dr. Komaroff, however, is convinced that the chronic fatigue syndrome is ''a real, organic disease.'' although the Epstein-Barr virus may not be the prime cause. Such symptoms as swollen glands, fevers and sore throats ''are hard to attribute to a psychological cause,'' he said, and most patients describe a sudden onset of disease, suggesting an infectious agent.

''When a patient says, 'I was perfectly fine and healthy until one day I came down with what seemed like the flu, but it never went away, and I don't have any energy any more,' that's a very dramatic story,'' Dr. Komaroff said. ''It's not something you can dismiss as primarily a psychological disease.'' Women and Yuppies

The ailment strikes women twice as often as men, for reasons that are not understood. It also disproportionately afflicts nurses, doctors and other health personnel. The most visible victims are often professionals in fast-paced jobs, leading quipsters to dub it ''the Yuppie plague'' or ''Yuppie flu.''

Most experts believe the syndrome has been around for many decades, although some accounts have suggested that the disease might be a new illness sweeping through the population. The clinical symptoms have been described in the medical literature for at least half a century under such names as epidemic neuromyasthenia, myalgic encephalitis, Iceland disease, Royal Free disease and postviral fatigue syndrome. Outbreaks or epidemics have been reported in eight countries dating back to 1934. Between outbreaks, the disease has presumably existed in people scattered through the population.

''It's not new,'' said Dr. Stephen E. Straus, chief of the medical virology section at the National Institute of Allergy and Infectious Diseases, one of the nation's few experts on the illness. ''What these patients complain of has been described for over a century. In each generation, people have a different idea of what it might be and what might cause it. I don't think there's much evidence that it's expanding.''

Nobody knows how many people suffer from the syndrome. Gidget Faubion, president of the National CEBV (Chronic Epstein-Barr Virus) Syndrome Association, a patient support group based in Portland, Ore., estimates that tens of thousands of Americans have it, noting that her organization has attracted some 8,000 dues-paying members and distributed some 60,000 brochures to interested people in two years. But skeptics suspect that many of those people have mild fatigue that would fade into insignificance under intense medical examination.

In the the May 1 Journal of the American Medical Association, investigators at Brigham and Women's Hospital reported that an astonishing 21 percent of some 500 patients who came to the hospital seeking primary care over a six-month period had suffered severe fatigue for at least six months, accompanied by chronic sore throat, aching muscles and headaches. But other scientists find it hard to believe that one in every five medical patients is really suffering from chronic fatigue. Articles Generated Interest

The current round of interest in the syndrome was generated, in part, by two articles in the January 1985 Annals of Internal Medicine that pointed to the Epstein-Barr virus, which is the cause of acute mononucleosis and of two cancers that are rare in this country, as a possible cause of the chronic fatigue syndrome.

Some 90 percent of all adult Americans have been infected with the Epstein-Barr virus, most without being aware of it. The virus lies dormant in a small fraction of their white blood cells and salivary cells, ready to proliferate should the immune system weaken.

The two articles reported that a substantial majority of some 75 patients with persistent fatigue had high levels of antibodies to the Epstein-Barr virus. That did not prove that the virus was causing the syndrome - it might easily have been an effect of the illness, emerging after something else weakened the immune system - but the articles raised the provocative possibility that the virus might somehow be responsible.

That was enough for the commercial testing industry, however. One aggressive company, in a 1985 mass mailing, urged doctors to buy its Epstein-Barr evaluation kit because ''one can expect enormous benefits to patients who can finally be told of a possible organic basis for their illnesses which for so many patients would otherwise be interpreted as 'neurotic' with resultant feelings of self doubt.''

At the same time, the National CEBV Syndrome Association was formed, and ''physicians throughout the United States were inundated with requests to evaluate chronic fatigue,'' according to a medical review article published this year. 'They Were Walking In'

Concern over the syndrome was especially heightened by the Lake Tahoe outbreak, when two doctors in Incline Village, Nev., found that they were treating a large number of patients for chronic fatigue from late 1984 to late 1986 and that many had elevated levels of antibodies to the Epstein-Barr virus. But the significance of this outbreak remains in dispute, even after an investigation of 134 of the Nevada patients by C.D.C. epidemiologists.

''We had an understanding that over 100 patients were severely affected by this syndrome,'' Dr. Holmes, who headed the C.D.C. team, said in an interview. ''But when we got there we found that a lot of these people had undistinguished symptoms. They were continuing to work. They were walking in. They really did not have any major signs of illness.''

A report of the investigation, in the May 1 Journal of the American Medical Association, said that 15 of the sickest patients had elevated levels of antibodies not only against the Epstein-Barr virus but also against measles virus, cytomegalovirus and herpes simplex viruses 1 and 2. The report suggested that Epstein-Barr antibodies were of no value in diagnosing chronic fatigue syndrome and that ''the illnesses may not be caused by EBV.''

Still, Dr. Holmes said ''something striking'' appeared to be going on in some of the patients that was ''not a figment of these people's imaginations.'' He added, ''When you get called by people who say they run marathons and participate in iron man triathlons and now can hardly get out of bed or brush their teeth, you realize that something is happening.''

Dr. Komaroff's group has drawn more alarming conclusions from studying some 175 cases at Lake Tahoe. He said ''quite a number'' of the people have been fully or partially disabled, while only about a third say their conditions have improved.

Scientists are now pursuing a range of studies to decipher the puzzling syndrome. Dr. Robert Gallo, one of the nation's leading AIDS researchers, and his colleagues at the National Cancer Institute are seeking to determine whether a newly discovered virus, called HBLV, may be involved in some cases of the chronic fatigue syndrome. Dr. Komaroff and his colleagues are studying how victims of the syndrome fare over time.

Dr. Straus and his colleagues at N.I.H. are exploring possible treatments for the disease; initial results from use of the drug acyclovir showed that it was no better at relieving symptoms than an inert pill. And investigators at a small number of medical centers are exploring other treatments, seeking diagnostic ''markers,'' and working on a vaccine for the Epstein-Barr virus.