IOTA, LA. -- It was a mystery woven deep into the local lore in Cajun country: Every so often, a child born healthy would degenerate into a "lazy baby" and slowly die.

No one realized that in rural Louisiana, where the same families have intermarried for generations, people were unwittingly passing around the gene for Tay-Sachs disease, an inherited and untreatable brain disorder that results in early death.

That discovery came one painful clue at a time with babies like Jandy Leger. At 7 months, Jandy laughed a long, delighted squeal. Her mother would say, "Dance for me," and Jandy would hold Gina Leger's hands as she twisted and wiggled a baby's jerky ballet. To the Legers, the brown-haired, wide-eyed Jandy was their "little angel."

In the 2 1/2 years since, Jandy's parents have watched as the deadly disease has taken life from their little girl one day at a time.

Now Jandy lies in bed, her face propped on a pillow, neither restful nor able to move. She can't talk or eat or play. She can't even cry.

Gina Leger soothes her daughter, then gently coaxes, hoping for any hint of a smile.

Most Tay-Sachs children die before they are 5, and doctors have told the Legers that Jandy could die tomorrow, in a month, maybe next year.

"What I wouldn't give to hear her laugh again," Gina Leger, 24, says sadly.

The immeasurable private tragedy is also a community problem.

Jandy Leger is one of at least four Tay-Sachs children diagnosed in the tiny Iota area during the last several years -- a remarkable number for a disease so rare that it only strikes about a dozen babies annually in the entire U.S.

As one family with a Tay-Sachs baby learned about another, they grew more and more alarmed. So did the residents in this town of 1,250 surrounded by rice and soybean fields, where residents say just about everyone is related in some way.

Traditionally, Tay-Sachs disease is associated with Eastern European Jews, known as Ashkenazis. In the mostly Catholic Cajun farming communities around Iota, doctors didn't even test for it. Most people had never heard of it.

Once told, people began to fit pieces together.

Many families recalled the "lazy babies" they had wondered aboutchildren with the same debilitating symptoms, who had died at the age of 3 or 4.

"I think they probably were Tay-Sachs babies," says Nancy Pousson, an Iota resident who remembers the story of a "lazy baby" in her grandmother's family. "The families didn't know better, the doctors didn't know, and the families didn't bring their children to doctors then."

Numbers bear out the notion that Tay-Sachs has been a fact of life here far longer than it has been recognized.

Emmanuel Shapira and his team at Tulane University in New Orleans tested 230 Iota residents last summer and found that 17, a stunningly high number, were carriers of the recessive Tay-Sachs gene.

A child must receive a Tay-Sachs gene from both parents to develop the incurable disease, which shows up just after infancy.

In Iota, that risk is relatively large. The number of carriers identified in the July screening factors out to 7.4 percent -- nearly double the carrier rate among Jews and 25 times that of the general population, Shapira said.

It will take more testing to know for sure whether the high incidence also will be found in the larger Cajun countryside. Shapira says thousands more people should be screened, and certainly those couples who want to have children.

In southern Louisiana, some already have taken to calling Tay-Sachs "the Cajun disease."

What isn't certain is how the gene made its way into the Cajun bloodlines.

At first, some presumed that the gene was carried into the Iota area by French Canadians, forebears of the Cajuns. Those who live along the St. Lawrence River are among the highest non-Jewish carriers of Tay-Sachs.

But research done by Miriam G. Blitzer and others at the University of Maryland in Baltimore has shown that the Tay-Sachs gene mutation found in Cajuns is identical to that found in the Ashkenazi Jews.

From there, it's all speculation.

Shapira and another doctor have pointed to historical works that suggest some Germans who helped settle the Iota area in the early 1700s were Ashkenazi Jews. Shapira also theorized that Spaniards hiding their Jewish heritage during the Inquisition may have carried the disease to the United States.

Blitzer and her associates are studying the gene's origins in the Iota area, but they have not yet completed their work and declined to discuss preliminary findings.

Whatever the gene's route of arrival, intermarriage and genetic isolation enabled carriers -- and Tay-Sachs children -- to become prevalent, experts say.

"If you have a group of people who marry among themselves, then the frequency of having one gene that multiplies among itself is much higher," said Shapira, who pointed out that the same explanation may be used for the high rate of sickle cell anemia in blacks and cystic fibrosis among whites.

Among Jews nationwide, testing for Tay-Sachs has become routine. The incidence has been diminished so much that, even though Jews are frequently carriers, more non-Jews have Tay-Sachs babies.

Unraveling the mystery of Tay-Sachs here started with Maigon Nicole Abshire, a blond, blue-eyed baby born on Christmas Eve 1985.

Maigon was 13 months old when a neurologist told her parents in 1987 that she had Louisiana's only known case of Tay-Sachs disease.

The Abshires were devastated and bewildered as they watched her slowly slip away. She grew mostly paralyzed, almost blind and was stricken with seizures.

Children like Maigon lack a vital enzyme that helps break down fatty deposits in the nerve cells of the brain.

Without the enzyme, the brain can't function properly, and the central nervous system deteriorates. A child becomes blind, deaf, unable to swallow, paralyzed and mentally impaired.

As David Abshire worked seven days a week to support staggering medical bills and his wife Renee became a round-the-clock nurse, the couple heard that an old friend of David's at Iota High School had a child like their daughter.

Renee Abshire wanted to know more. She called Michelle Brown, and Brown said, yes, her son, Brenton, was suffering from Tay-Sachs. Abshire was shocked.

But there was more. Six months later, Gina Leger, Jandy's mother, called Abshire. A year after that, Leger learned about Gail McClelland, whose son Jordan had the disease.

Three of the four families have roots in Iota and live in the rural areas around it; the Legers live in the nearest city, Lafayette, about 30 miles east of Iota. As the families began to find one another, Maigon Abshire died shortly after her third birthday. So did Brenton Brown, who was 5 1/2. "We decided we had to do something," Renee Abshire recalled.

A month later, they formed the Louisiana chapter of the National Tay-Sachs & Allied Diseases Association as a support group for other parents. Soon afterward, they drove to New Orleans to discuss their cases with Shapira.

From that meeting, the Iota screening was arranged.

"It's been told to us since we were young, 'Don't intermarry because of what could happen to your children,' " says Abshire, now president of the Louisiana Tay-Sachs association, "but I think we're seeing now what really could happen . . . when you marry possibly your cousin."

It doesn't always take marrying a cousin, either. Abshire was a minister's daughter from Texas when she married her husband. Gina Leger doesn't believe her family was even remotely related to her husband's.

For many, knowing who is a carrier poses difficult choices. If two spouses are carriers, their child has a one-in-four chance of getting the disease.

Tay-Sachs may be diagnosed early in a pregnancy -- but in conservative, Catholic southern Louisiana the choice to abort is seldom considered an option.

"It's a personal decision, but it's not an option for me and my husband," says Abshire, who, at 26, will not consider bearing another child unless an artificial fertilization method can assure that both egg and sperm carry no Tay-Sachs gene. More Information

Genertic screening for Tay-Sachs is offered by appointment at Johns Hopkins Hospital in Baltimore and Children's National Medical Center in the District. Details can be obtained by calling the National Capitol Tay-Sachs Foundation at 301-279-5878.