The disappearance of rheumatic fever in the U.S. used to be one of the good news stories in medicine. Though doctors are at a loss to explain why, incidence of the heart-scarring disease had dropped to near-zero by the mid-1980s -- so low that most young physicians have never seen a case.

But now, and for reasons as mysterious as the initial decline, the disease seems to be making a comeback.

In August, doctors from the Children's Medical Center in Akron described 23 cases of rheumatic fever seen at their hospital in 1986. In the previous decade, they state in the Journal of Pediatrics, there were two to three cases a year.

In July, researchers in Columbus, Ohio, reported 40 cases in two years, compared to a total of only 34 during the previous decade. Dallas, Denver and Pittsburgh have also checked in with outbreaks.

The biggest numbers come from Salt Lake City, where since January of 1985 Dr. George Veasy of Primary Children's Medical Center has seen more than 120 children with rheumatic fever -- eight times the incidence rate of the previous decade.

Says Dr. Edward Kaplan of the University of Minnesota, who runs a World Health Organization laboratory that deals with rheumatic fever, "I think this represents a significant change in the epidemiology of this disease."

Rheumatic fever, at least in the last couple of decades, has been rare in the United States but common in the Third World. The disease is still responsible for almost half of all heart disease in developing countries, according to Kaplan.

It primarily strikes children between the ages of 5 and 15 and is an occasional follow-up to sore throats caused by streptococcus bacteria. Only a few percent at most of strep sore throats evolve into rheumatic fever, and many children get rheumatic fever without having had a sore throat first. Like many infectious diseases, strep throat and rheumatic fever are more prevalent in the winter months when children remain indoors and in close quarters.

In cases that are preceded by strep throat, the problems generally begin a few weeks after the sore throat has gone away. Rheumatic fever is actually a syndrome -- it manifests itself in many ways. It can show up as arthritis, an inflammation of the heart muscle, bumps under the skin, muscle spasms or a rash. In addition, there are several minor symptoms, including fever and joint pain.

While the symptoms generally disappear, the problems don't end there. About one third of children who have had a bout of rheumatic fever suffer damaged heart valves as an immediate result -- the valves that control blood flow within the heart don't open or close fully. As a result, they may be occasionally short of breath or have heart pain, dizziness or fainting on exertion.

And often heart valve problems show up years afterward. Some people eventually need their valves surgically replaced.

There are people who've had rheumatic fever who don't suffer obvious heart problems, but they remain at risk of another, more damaging encounter with rheumatic fever. To prevent a reinfection, most people who've had rheumatic fever are put on long-term, sometimes life-long penicillin therapy.

The body itself is the suspected culprit. Though not yet proven, most researchers believe that in the fight to rid itself of streptococcus, the immune system produces antibodies that attack the body's own tissues.

There are no national figures available for the incidence of rheumatic fever in the first half of this century, but it was common enough that many large cities had entire hospitals especially dedicated to caring for children with rheumatic fever. Baltimore, for example, had Happy Hills Convalescent Hospital, opened as a 30-bed hospital in 1922. By 1929, it had expanded into a 62-bed hospital. Now called Mt. Pleasant Pediatric Hospital, the institution saw its last patient with rheumatic heart disease in 1975; it now cares for children with other chronic diseases.

During each year from 1935 to 1949, 65 per 100,000 children between the ages of 5 to 14 in Rochester, Minn., got rheumatic fever. The level dropped to 41 a year during the time from 1950 to 1964 and to nine a year from 1965 to 1978.

While people stopped collecting statistics when the disease became so rare, several studies have shown rates in several communities in the early 1980s below one in 100,000.

In Washington, according to pediatric cardiologist Dr. Lowell Perry of the Children's Hospital National Medical Center, no increase has been seen.

The triumph over rheumatic fever, whether temporary or not, is not a victory doctors can claim for themselves. The disease began its decline as early as the 1920s and 1930s, well before the widespread introduction of antibiotics that followed World War II. And unlike other now uncommon childhood diseases like measles and polio, there's no vaccine against rheumatic fever or strep throat.

The general belief is that an improvement in living conditions -- less crowding, improved sanitation and better nutrition -- somehow combined to thwart the spread of the disease. If true, it explains why the disease is still so common in the Third World.

The re-emergence has researchers scrambling for an explanation. Because rheumatic fever has appeared in several widely separated areas, they don't think it's just a statistical anomaly. Says Kaplan: "It shows that penicillin and improved care are not the total answer to rheumatic fever."

Last year, the Centers for Disease Control did a rough national survey to get an idea of the problem. According to CDC epidemiologist Dr. John Spika, in the 27 states that don't have an official surveillance system, three reported a possible increase in cases. In the 24 states that have a surveillance system, eight reported some suggestion of an increase.

"However," says Spika, "you have to be careful. The numbers are quite small. It's nothing on a grand scale. We're still not approaching the levels even of the early 1970s."

The disease itself is somewhat different from the earlier version. Rheumatic fever used to hit primarily lower-income families; all the current outbreaks have clustered in middle-income families. And where about one half to two thirds of cases are thought to follow a strep throat, in Pittsburgh only one quarter did and in Utah it was only one third.

There also seems to be more frequent heart involvement -- in Utah, says Veasy, 60 to 70 percent of the children suffer heart damage after just one episode.

While there is no way to totally prevent the disease, the odds can be cut by caring for all strep throats. "Doctors have got to be alerted to this," says Veasy. "And parents need to take sick kids in {for a checkup}."

The next step is a throat culture. If streptococcus is found, the doctor will probably recommend either a single penicillin injection or a 10-day course of oral penicillin. The 10 day treatment is key; Kaplan recently showed that five days of penicillin won't kill all the bacteria.

The final mystery in the disease is why -- why, after so many years, is it coming back? Some researchers suggest, because rheumatic fever clusters in families, that certain people have genes that make them more prone to the disease. But there's little explanation as to why there'd suddenly be more of those people around.A more popular theory has to do with the bacteria themselves. Several laboratories, Kaplan's included, are looking at the strains involved in the outbreak to see if they have something new or different -- some protein on their surface, perhaps. Such a mutant strain could possibly be more able to spark the disease.

Or it could be a combination -- a new or re-emergent strain preying on genetically susceptible people.

While scientists work out what's causing the current outbreaks and determining whether they are spreading, there is no need to panic, they emphasize. "I don't think it's justified to sound the alarm and say we're in a dangerous situation, because we're not at this time," says Spika. But, he adds, it is a disease worth watching.

Says Dr. Alan Bisno, one of the authors of a 1983 paper that called rheumatic fever "a vanishing disease," says: "It could be we'll see a few clusters that die out, or it could be more general. Only time will tell." His suggested solution? A strep vaccine, which would be a boon not only in the U.S. but worldwide.

Joanne Silberner covers health for U.S. News & World Report.