The journey that would take me to the epicentre of the “Alzheimer’s Plague” began in a crowded bar in the Colombian coastal town of Cartagena de Indias. It was there that I met by chance the legendary writer Gabriel García Márquez. He was at the centre of a long table, surrounded by admirers, but with a lost and vacant expression which reminded me of my father, who had died of Alzheimer’s in 1998. Soon afterwards, when I finally had the opportunity to talk to him, I would discover that his own memories were rapidly fading, and coming increasingly to dwell on the Colombian river that had marked his childhood, the Magdalena. He encouraged my long-held ambition to travel up this river, while also turning my mind to the subject of memory.

Re-reading for the first time since a teenager García Márquez’s most famous novel, One Hundred Years of Solitude, I dwelt on its description of the memory disease that strikes the imaginary village of Macondo. Its symptoms are insomnia followed by such acute memory loss that all objects have to be labelled with their names and functions until eventually words themselves cease to have any meaning. I had previously interpreted this part of the book as a commentary on Colombia’s ability to forget its past. But, after my encounter with the author in Cartagena, I would see it as an example of his extraordinary prescience: he had foreseen not only his own condition in later life, but also a form of dementia that would only be identified a few years after the book’s publication in 1967.

Later, as I prepared to embark on my Magdalena journey, I encountered further uncanny parallels with the novel after reading about a cluster of Colombian towns and villages whose inhabitants were fast falling victim to a rare strain of early onset Alzheimer’s. Until very recently these inbred communities, situated high above the river’s valley, in the Colombian region of Antioquia, had been isolated from the outside world by a combination of their remoteness and the former heavy presence of guerrillas, drug-traffickers, and paramilitaries. Over the past few years, however, this once obscure corner of Colombia has been drawing a growing number of scientists searching for answers to Alzheimer’s in general. I decided to investigate.

I travelled to the area by way of the regional capital of Medellín, a place that has shaken off its image as the murder centre of the world to become a showcase of the new Colombia. Vibrant, full of spectacular new buildings, and set in a lush and temperate valley, Medellín is also the city where much of the research on this Antioquia variant of Alzheimer’s is being carried out. The person in charge is Dr Francisco Lopera, who has finally managed to secure for his project the funding and collaboration of Americans previously put off by Colombia’s violent reputation.

Dr Lopera, who has just turned 60, and is more in demand than ever, talked to me as if he had all the time in the world to do so. He told me how his interest in the mind went back to a childhood fascination with flying saucers, and to the realisation that these were products of the imagination.

After flirting with psychiatry, he ended up as a neurologist who, in 1982, experienced a turning-point in his career after being visited in Medellín by a family from a village called Belmira. They had brought in a 49-year-old farm labourer who, over the past four years, had lost all his words, memories, and control over his actions. He had been reduced to occasional outbursts of crazed laughter, and to the look of someone who appeared to be “staring at infinity”. The family held out no hope for a cure for him, but had come to Dr Lopera largely out of desperation, to know if there was anything that could be done to alleviate his symptoms. On being questioned further about him, they revealed that many of the man’s relatives had suffered from the same malady, including his mother, grandmother and three of his maternal aunts. Instead of thinking of his disease as a hereditary one, they blamed it on an ancestral curse.

Dr Lopera kept the man for observation for several days, but then, realising that nothing could be done, released him with the diagnosis of “pre-senile dementia”. He was struck by the patient’s youth, by the genetic component of his illness, and by the case’s remarkable similarity with one studied in 1906 by Freud’s pupil, Alois Alzheimer. However, it was not until the early Nineties, when several other such cases came to light in nearby villages in Antioquia, that Dr Lopera became confident of having uncovered a major local phenomenon of potentially global implications. He would later call this the “Alzheimer’s Plague”, though a definitive diagnosis of Alzheimer’s could only be made in 1995, when the first of now dozens of precociously shrunken brains was presented for analysis to Medellín University.

“We have still no idea of what causes conventional Alzheimer’s,” said Dr Lopera, “but the form of the disease we’re seeing here in Antioquia is a genetic disorder produced by changes to chromosome 14.” This so-called “Paisa variation” has been recognised so far in 25 families in the area, all of whom seem to have been descended from a single Basque who had settled in a village called Angostura at the end of the 18th century.

“These families, amounting today to around 5,000 members, constitute the world’s largest research pool of potential Alzheimer’s sufferers. About half of these people could end up developing the disease.”

Despite the differences between this inherited form of Alzheimer’s and the mainstream one afflicting the elderly generally, the brain lesions in both types of the disease were found to be identical. This persuaded Dr Lopera and his team that by closely monitoring those likely to succumb to the “Paisa variation”, a greater understanding could be had of how Alzheimer’s develops, and a drug could then be tested that might delay the onset of the full-blown illness. After studies were made on over 800 people from the Antioquia region, a pioneering drug inhibiting the accumulation of amyloid protein (a feature of those acquiring the disease) was tried on volunteers. Its effectiveness, according to Dr Lopera, should be known within five years.

Dr Lopera offered to drive me at dawn to the mining town of Yarumal, at the heart of the “Alzheimer’s district”. We travelled through a sodden, foggy landscape of pines, pastures and horse-riding cowboys disappearing into the gloom. The scene provided an appropriate background to a conversation peppered with Dr Lopera’s stories about all the local folk superstitions surrounding the disease. Known popularly as La bobera (“The stupidity”), it was blamed by some people on a village priest who had put a curse on his parishioners for stealing from the collections box. Others believed you caught the disease from touching the bark of a certain rare tree.

The conditions in which Dr Lopera and his team had undertaken their original field research were certainly different to anything his current American collaborators could have known. I had an insight into these soon after reaching Yarumal, a town clinging to the steep slopes of a verdant mountain. Among the first people I met there was one of the doctor’s most faithful assistants, Lucía Madrigal, a tough Spaniard who had been kidnapped in the Nineties while collecting blood samples in a nearby village. “I don’t mind what you do to me,” she had told her assailants, “but please be careful with the samples.” They had kept these cool in a river before releasing her after eight days. The experience, she said, had not deterred her from returning to the area, and she had done so only a few months later, after one of the kidnappers had contacted her to ask for help for his mother, a victim of the “memory disease”.

Many of the other local cases of Alzheimer’s had come to light thanks to the zeal of Dr Lopera’s driver Antonio. Now retired, he spoke to me about his excitement whenever he heard of some other family with the disease’s symptoms. “I’ve found another one!” he would ring to tell Dr Lopera, as pleased as if he had had a sighting of a rare bird.

I was warned that Yarumal was a depressing place, but the warmth of its inhabitants counteracted any negative impressions, and kept my spirits up when the time finally came to see one of the Alzheimer’s families. I was feeling voyeuristic about doing so, but was persuaded otherwise by the young and energetic local health counsellor Gloria Gómez, who stressed how the visits of outsiders provided a necessary relief to the overworked family members who had to look after their sick ones. From Gloria I found out that only one of the area’s Alzheimer’s victims had been sent to the local old people’s home. Whereas putting the demented into a home was a common practice in a city such as Medellín, it was almost unthinkable in rural Colombia, where mothers, daughters and sisters became full-time carers.

The forbearance and strength of these people became apparent on visiting the household of Laura Cuartas, who lived with four of her children in a dark, cramped house at the bottom of one of Yarumal’s near vertical streets. Her husband had died of Alzheimer’s after having fathered 13 children, four of whom had contracted the disease in their forties. One of these, who also had Parkinson’s, was being cared for in Medellín. The three others were totally dependent on Laura and on one of the unaffected sisters. The most recently diagnosed was a laughing and still active man who confirmed one of the particular characteristics of this type of Alzheimer’s: a complete lack of awareness of having the illness. His two other ill siblings, a man and a woman, had reached the disease’s final stage, though this was apparently a stage that could last for years. Their bodies had started forgetting how to function, and they lay motionless on beds, like living corpses, with eyes wide open, and mouths incapable of speech.

“The carers are the ones who really suffer,” Gloria told me as we left Yarumal and drove in a crowded van to the village of Angostura, the cradle of the “Paisa variation”. Two men, who had been following us, got into the back of the van and later introduced themselves as secret policemen protecting me from potential kidnappers. Soon their banter would be contributing to the almost holiday mood that began building up as the sun emerged to reveal a colourful landscape of distant mountains, flower-lined verges, and fairy-tale-like houses. Angostura itself was how I had once imagined García Márquez’s Macondo to be like – a near intact colonial village centred on a broad, cobbled square covered with images of the “Santo Marianito”, a miracle-working local priest in the process of being canonised.

Gloria proposed taking me to see more afflicted families, but we ended up instead having a long lunch. Eating was usually the last pleasure after memory had gone, said Gloria before discussing the possibility of an Alzheimer’s cure, and the conviction of so many of Antioquia’s inhabitants that one would soon be found. Such faith in the anti-amyloid drug seemed to me unduly optimistic when the pharmaceutical industry, tired of endless costly failures to contain Alzheimer’s, is almost on the point of giving up on the disease. But then I remembered Macondo, and how a magician’s potion had suddenly “turned on a light” in people’s memories, and brought the plague there to an end. The world, with its present population of 36 million Alzheimer’s patients, can only hope that García Márquez’s remarkable prophetic powers will once again prove true.

'The Robber of Memories: A River Journey Through Colombia’, by Michael Jacobs (Granta, £16.99), is available to order from Telegraph Books at £15.99 + £1.35 p&p. Call 0844 871 1516 or visit books.telegraph.co.uk

This article also appeared in SEVEN magazine, free with the Sunday Telegraph. Follow us on Twitter @TelegraphSeven