NEW YORK -- Planning to take advantage of these last few glorious autumn weekends with a hike in the woods? Or a hunting trip? Be warned that doctors are growing concerned over a recently recognized disease that is increasingly common.

Lyme disease -- regarded rather condescendingly only a few years ago as a minor, localized problem -- is now becoming recognized as a worldwide health menace with potentially severe consequences for all whom it attacks.

Otherwise known as Lyme borreliosis, the tick-borne bacterial infection first came to public notice a dozen years ago as the cause of a sudden, mysterious outbreak in Lyme, Conn., of rash followed by arthritis and flu-like illness.

Now known to exist on every populated continent, Lyme was the subject of a recent international conference -- cosponsored by the New York Academy of Sciences and the New York State Health Department -- to discuss strategies for dealing with this baffling disease.

"This is a disease of global importance," microbiologist Jorge Benach of the New York State Health Department said. "It is an entity that can affect all organ systems. It is a disease that we deal with very poorly."

It is also a disease that can strike anew, whenever the victim is bitten by an infected tick. Unlike those of such better-known infectious diseases as measles and polio, the antibodies it provokes in the human body apparently confer no lasting immunity. Thus it is a difficult challenge for vaccine developers, although Dr. Russell Johnson of the University of Minnesota, who is using hamsters to try to develop such a vaccine, told the meeting that he is "fairly confident" it can eventually be done.

Moreover, apparent recovery from an initial Lyme infection can be followed by skin, heart, nervous system and other complications, months or years later. Even without subsequent tick bites, the disease can come back again and again, each time in a more serious form that is increasingly difficult to treat.

To confuse the picture further, antibiotic treatment of early Lyme disease seems to cure most patients, but whether it also will prevent the later manifestations of the illness is one of the many things about this disorder that is not yet known. Also worrisome is the possibility, yet to be thoroughly studied, that the disorder can be transmitted by infected blood. And, in one reported case, the possibility of transmission thorough an infected mother to an unborn child has been raised.

Lyme disease, in short, has many of the characteristics of an old menace, syphilis, which is not surprising since the germs that cause both diseases are closely related. Both are spirochetes -- corkscrew-shaped bacteria less than a ten-thousandth of an inch long.

In the old days, before antibiotics were found to be effective against syphilis, that disease was called "the great imitator" because of its propensity for mimicking other diseases. In a paper at this meeting, neurologist Andrew Pachner described Lyme as "the new great imitator."

Pachner, who studied the disease while on the medical faculty at Yale, has moved to Washington and opened a Lyme clinic at Georgetown University. At the conference, Pachner sounded a warning note to all who suffer, and seem to recover from, early manifestations of Lyme disease.

"Anyone suspected of having had Lyme disease should have a full neurological evaluation," Pachner said. "I would suspect that obvious clinical manifestations {of the illness} are just the tip of the iceberg in terms of how often the organism goes to the brain."

Pachner's point was bolstered by reports from several other scientists, both American and foreign, who presented papers at the meeting. For example, Dr. John Halperin of the State University of New York at Stony Brook noted that it is possible to have Lyme without ever developing either the antibodies in the blood that are diagnostic of the disease or its characteristic rash -- known as erythema migrans (EM) -- a telltale red spot that shows up at the site of a bite by an infected tick and then enlarges and spreads.

For this reason, Halperin reported, he and his colleagues do not rely solely on a past history of EM or blood tests when they see patients they suspect may have chronic Lyme disease. Nor do they automatically rule out chronic Lyme in patients who complain of severe fatigue, memory and concentration problems after they have recovered from a first attack, even though other symptoms -- such as facial paralysis, meningitis or shooting pains in the arms and legs -- are considered to be the "classic" delayed effects of the illness.

Instead, Halperin said, patients may also be tested with such methods as nerve function and brain wave studies, brain scans with CT (computerized tomography) and MRI (magnetic resonance imaging), spinal taps and tests of muscle coordination and memory that together can help determine whether their mental difficulties and unpleasant sensations are related to Lyme disease. If they are, it is Halperin's experience that these problems are "readily reversible with appropriate antibiotic therapy."

Late nervous-system effects of Lyme disease are another notable point of similarity with syphilis. In Stockholm, 75 patients with delayed neurological disorders from Lyme disease were treated between 1984 and 1986, a Swedish researcher reported at the meeting. Dr. Goran Stiernstedt from Sweden's Karolinska Institute described aftereffects that resemble multiple sclerosis, brain tumor, stroke, alcoholism and mental depression. Stiernstedt referred to Lyme in this manifestation as "neuroborreliosis," in reference to the germ that causes Lyme in all its manifestations.

Though Lyme disease is popularly supposed to have originated in the Americas (as syphilis did thousands of years ago), there is now some evidence that its germ may have been native to the Old World and come here later, possibly in a tick riding piggyback on a migratory bird or other animal.

Supporting the notion of an east-to-west migration of Borrelia was a British scientist's announcement at the conference that the disease, hitherto considered a rarity in the United Kingdom, is now thought to have existed there for many years. Blood samples taken from deer ranging the New Forest in southern England 20 years ago -- eight years before the arthritis-like outbreak among children in Lyme, Conn., gave the disease its present name -- have been found to contain antibodies to the Borrelia germ.

A disease now recognized as Lyme has been known since before World War I on the continent of Europe, where it was regarded as a neurological disease rather than a rheumatic or arthritic one. Only five years ago was the disease's worldwide cause identified as a Borrelia spirochete by Dr. Willy Burgdorfer of the National Institutes of Health's Rocky Mountain Laboratory in Hamilton, Mont., whose name gives the Lyme germ its species designation, burgdorferi. Unknown in America until the 1960s and unnamed until the Lyme incident of 1975, the most menacing aspects of the disease have only begun to become apparent. Microbiologist Benach, a cochairman of the international meeting, predicted that the nervous-system involvements seen in Europe will become increasingly common here in years ahead. The time is ripe, he said, for secondary and tertiary Lyme to start appearing here.

For all its public-health implications, Lyme remains a medical novelty in many parts of the United States, although it now has been reported in 32 of the 50 states, including Maryland and Virginia. Dr. Carol Ciesielski, an epidemiologist with the federal Centers for Disease Control, told the conference that 80 percent of known cases have occurred in seven states in two regions: Massachusetts, Rhode Island, Connecticut, New York and New Jersey in the East; Minnesota and Wisconsin in the Midwest. A much smaller focus of Lyme disease infection is in the coastal counties of California north of San Francisco.

The disease is traveling south along the Atlantic seaboard, catching many medical practitioners by surprise. Dr. William Massey III, an internal medicine specialist in Williamsburg, Va., told a reporter at the conference that he had seen two classic Lyme disease cases this summer after reading a newspaper article in May.

Even in the Northeast, where Lyme has been commonplace for a decade, many physicians seem to be unfamiliar with its symptoms. Late this summer, Benach said, a doctor in one of the disease's hottest spots told one of his patients that "there is no such thing as Lyme disease."

Exactly how much Lyme disease is going around is a topic of some debate. The CDC estimates close to 6,000 cases since it started a surveillance campaign in 1980, but almost 1,500 of these were recorded last year alone. Many experts think the true numbers are at least three times higher. But they despair of finding out until more state legislatures act to require doctors to report all the cases they see, as they are already required to do for Rocky Mountain spotted fever, another tick-borne illness that is far less common.

Meanwhile, virtually all specialists agree on two points: First, Lyme disease is shockingly under-reported, and, second, it is proliferating both in numbers and in areas affected.

William Hines and Judith Randal are Washington writers.