YARUMAL, Colombia - In this rough-hewn mountain town, an old woman found herself diapering her middle-age children.

At frighteningly young ages, in their 40s, four of Laura Cuartas’s children began forgetting and falling apart, assaulted by what people here have long called La Bobera, the foolishness. It is a condition attributed, in hushed rumors, to everything from touching a mysterious tree to the revenge of a wronged priest.

It is Alzheimer’s disease, and at 82, Mrs. Cuartas, her gray raisin of a face grave, takes care of three of her afflicted children.

One son, Dario, 55, babbles incoherently, shreds his socks and diapers, and squirms so vigorously he is sometimes tied to a chair.

A daughter, Maria Elsy, 61, a nurse who at 48 started forgetting patients’ medications, and whose rages made her attack a sister who bathed her, is a human shell, mute, fed by nose tube.

Another son, Oderis, 50, denies that his memory is dying. If he gets Alzheimer’s, he says, he will poison himself.

For generations, the illness has tormented these and thousands of others among a sprawling group of relatives: the world’s largest family to experience Alzheimer’s disease. Now, the Colombian clan is center stage in a potentially groundbreaking assault on Alzheimer’s, a plan to see if giving treatment before dementia starts can lead to preventing it altogether.

Most family members come from one Andes region, Antioquia. Geography, and Basque ancestry, have isolated people here. Over three centuries, many in this clan of 5,000 people have inherited a single genetic mutation guaranteeing that they will develop Alzheimer’s.

Large families, and intermarriage, have accelerated the spread. Mrs. Cuartas’s fourth debilitated child, in Medellin, Carlos Alberto Villegas, a former livestock trader and guitar serenader now often fed by baby bottle, married a distant cousin. His mother-in-law is an addled ghost; three of his wife’s 11 siblings, so far, are developing dementia.

With Alzheimer’s in both parents’ families, Mr. Villegas’s three children could face extraordinary risk. One, Natalia, 22, asks: “How long have I got, till I’m 35? There’s no way out.”

This form of Alzheimer’s, early-onset, was once considered too different to provide clues about far more common late-onset Alzheimer’s, which has unknown causes and mostly affects people over 65.

But it turns out that both forms produce nearly identical brain changes and symptoms. Now, scientists will test as-yet-unproven treatments on Colombians genetically destined for Alzheimer’s but not yet showing symptoms. They will give a to-be-determined drug or vaccine and see if it prevents memory loss or brain atrophy.

Alzheimer’s has repeatedly resisted attempts to treat it. Current drugs, for people who are already impaired, show little benefit.

With Alzheimer’s afflicting 530 million people worldwide, numbers that some predict will double or triple by 2050, “we can’t wait to try to do prevention until we are absolutely certain what causes” the disease, said Neil Buckholtz, chief of dementias of aging at the National Institute on Aging. “This public health emergency,’’ he said, is “just going to get out of control if we don’t do something.”

Scientists are recruiting participants in Colombia, hoping to start testing next year. Family members without symptoms, roughly 38 to 45 years old, will be gene-tested, given treatment or placebos, and monitored with memory tests, brain scans and other measures.

The treatment will attack a protein, beta-amyloid, considered a culprit by many scientists because it creates plaques, deposits between nerve cells. The treatment could be a drug that destroys or prevents plaques, or a vaccine that contains or encourages production of anti-amyloid antibodies.

Scientists are recruiting drug-industry sponsors for the $50 million trial They currently have $8 million from donors.

Some companies envision testing new treatments; others envision recovering investment in therapies that failed with already-demented people.

Researchers will also explore when Alzheimer’s begins, and why, examining which brain changes, or biomarkers, precede symptoms. If brain shrinkage, brain activity or beta-amyloid levels are identified as definitive biomarkers, doctors may be able to treat those, the way they treat cholesterol to prevent heart disease.

Scientists recently rode into Angostura, ancestral home of many family members.

Villages like Angostura had often been too dangerous. Once, a nurse, Lucia Madrigal, was kidnapped by guerrillas while collecting blood for gene testing. “I do not care how long you retain me,” she said, but “take care of the samples.”

She was released eight days later (guerrillas chilled the blood in a river), but the team she belonged to stopped visiting some villages for years.

During that time, Ms. Madrigal visited Angostura only when asked by a guerrilla boss. His mother had Alzheimer’s.


By PAM BELLUCK