Lyme disease, which is beginning to rival sunburn as summer's chief health threat, is turning out to be far more widespread and far more serious than had been thought last summer.

The tick-borne infection first turned up in a few cases in Lyme, Conn., 15 years ago but has exploded across 43 states to become the most common disease in the United States spread by an insect, spider or other arthropod. Now Lyme disease is spreading rapidly through most of Europe and the Soviet Union and cases have turned up as far away as Japan and downtown London.

Although the dominant mode of transmission is through the bite of an infected deer tick, there are now reports that some people got the disease from deer blood and the urine of an infected rabbit.

Moreover, doctors are learning the disease can be far more serious than was realized. Once thought to cause little more than flu-like symptoms, rash and joint pains, Lyme disease has in some cases led to severe heart problems, permanent joint disability and dementia.

Now there are reports of stillbirths to women infected with Lyme disease while pregnant, according to Thomas Schwan, acting head of the arthropod-borne disease section of the Laboratory of Vectors and Pathogens for Rocky Mountain Laboratories in Montana.

Lyme disease is a threat in any habitat where there are deer, including the Washington area. Maryland authorities counted 105 cases last year and 13 so far this year. Virginia recorded 59 last year and nine this year. Even the District, which has deer in Rock Creek Park and along the Potomac, had two cases last year and two so far this year.

While scientists disagree about whether Lyme disease can be treated during the later stages of infection, all agree that in the early stages its progress can be halted with antibiotics. But early symptoms, which often include a ring-shaped rash and fever sometimes are absent, and blood tests of infected humans can be negative. More than 20 percent of the more than 5,000 cases reported in 43 states last year had no early symptoms, and only 17 percent of those infected recalled having been bitten by a tick.

Scientists link the epidemic to growth in the number of ticks that feed on a deer population that has been increasing throughout the temperate world. White-tailed deer are the preferred host of the Lyme disease tick, a species that is much smaller than the dog tick. Deer ticks, about the size of sesame seeds, transmit the disease-causing spiral-shaped bacterium, called a spirochete, when they bite animals or people for a blood meal.

"You were really hard pressed to find ticks with Lyme disease 10 years ago," said Durland Fish, director of Medical Entomology Laboratory for New York Medical College. "We've been measuring an increasing tick population since 1984, growing at a very, very high rate. Now they're extremely abundant."

But infected ticks are no longer thought to feed only on deer. They have been found feeding on many warmblooded animals and some scientists speculate that Lyme disease has been transmitted by infected arthropods other than ticks.

Willy Burgdorfer of Rocky Mountain Labortories, who first discovered the spirochete in 1981 and after whom it is named Borrelia burgdorferi, said he was infected after he was accidentally squirted in the eye with infected rabbit urine. Robert Lane, an entomologist at the University of California at Berkley, said field biologists handling slain deer may have contracted the disease from infected blood, explaining that Lyme disease bacteria have the unusual ability to penetrate unbroken skin. Infected birds, too, are suspected as possible transmitters of the spirochete.

"There have been numerous collections of ticks from birds that are infected with the spirochete," Schwan said, explaining that it's not known whether all infected ticks found on birds actually acquired the bacteria from them. "At this point, there is only one isolation from the bird where the bacteria has been isolated and grown up in culture and specifically identified as the Lyme bacterium."

Ixodes ticks have a two-year life cycle and it is critical that during its two immature stages -- larval and nymphal -- the ticks feed on the same host species, predominantly white-footed mice. Spirochetes are transmitted from infected nymphs to mice during early summer and then from infected mice to a new generation of larvae during late summer.

The only proven method for controlling the spread of Lyme disease, developed by the Harvard School for Public Health, uses pesticide-treated cotton balls in cardboard tubes. The tubes are scattered in tick-infested areas. Mice gather the cotton to line their nests, where the pesticide rubs onto their fur. When larval ticks feed on the mice, the pesticide kills them.

"Borrelia burgdorferi has been demonstrated in other species of ticks and in mosquitoes and deer flies, but only ticks of Ixodes ricinus complex seem to be important in the transmission of the spirochete to humans," Allen C. Steer wrote in the New England Journal of Medicine last August. "Ticks have been found on 30 types of wild animals and 49 species of birds. If deer are removed from an established focus, the tick may be able to adapt and survive on other animal hosts."

"I don't think there is any reason for panicking," Burgdorfer said. "If you concentrate on developing specific diagnostic tests then I think you can treat this disease in the early stages quite readily with antibiotics."

Doctors recommend that people in areas where deer are prevalent check themselves for ticks daily. Ticks should be removed by any convenient method such as fingers or tweezers. "If the tick is attached for more than 24 hours, you're at high risk of being infected."