The human animal derives its humanity from language and memory. What are we, without memory ? The short answer is: wild beasts.

Memory gives us personality, emotional intelligence, family relations, and community. Memory anchors us in space and time. It defines the parameters of existence. Paradoxically, it might even confirm the futility of existence.

Dementia, in the broadest sense, lays an axe at the root of memory, creating that “bare, forked animal”, unaccommodated man. “Keep me in temper,” exclaims King Lear before his final breakdown, “I would not be mad.”

Madness comes in many guises, but the cruellest manifestation that’s hitting the headlines today is the affliction named after the German doctor who first identified its most virulent strain in 1906, Professor Alois Alzheimer.

The biology of the ageing brain remains among the greatest enigmas of neuroscience. For several decades, the German psychoanalytic establishment seized on the mysterious nature of the disease to subordinate “insignificant” biological explanations of dementia to broader, Freudian interpretations. Until the 1960s, Alzheimer’s was at once neglected and controversial. If no one could agree about its fundamental symptoms, many others disputed its causes. Slowly, as a result of improved brain-mapping, and the identification of “plaques and tangles” in the geriatric brain as a source of dementia, Alzheimer’s emerged as the global epidemic we now recognise.

Alzheimer’s has become a new plague, threatening the world’s population with a global strike rate of one every four seconds. In the UK, there are now more people with the disease than live in the city of Liverpool. Six million inhabitants of the EU and 4 million Americans have it, figures that are projected to double by 2030. So bad is the outlook that the WHO has declared dementia a global health priority.

It has become the salient fact of 21st-century life that, with an ageing world population, Alzheimer’s will overtake cancer as the second leading cause of death after heart disease. We’re at a point, writes Joseph Jebelli, at which “almost everyone knows someone – a family member or friend – who has been affected.”

Jebelli, a young British neuroscientist, has greater cause than many to make this claim. As a boy, he watched his grandfather acting strangely, before descending into the abyss of dementia in which he could no longer recognise his family. Jebelli’s testament, In Pursuit of Memory, is a moving, sober and forensic study of the past, present and future of Alzheimer’s from the point of view of a neurologist who has lived with the disease, at home and in the lab, from a very young age.

Jebelli’s timely analysis is a reminder that, in recent years, Alzheimer’s or other forms of dementia have not merely devastated the lives of millions, they have destroyed the retirements of Harold Wilson, Ronald Reagan, Charlton Heston, and Margaret Thatcher, killed Terry Pratchett, and claimed Glen Campbell and Iris Murdoch among its victims. The lineaments of this fate were recently dramatised in the Oscar-winning film Still Alice, starring Julianne Moore.

WH Auden once compared death to “the rumble of distant thunder at a picnic”. The stages of Alzheimer’s occur as storm clouds on the horizon of a perfect summer’s day. The initial symptoms – flashes of anger; occasional forgetfulness – are often so slight that even doctors can misdiagnose them. As the disease takes hold, it becomes clear that something terrible is happening to the patient’s brain (repetitive questions; the inability to recognise friends and family).

Finally, as Alzheimer’s ignites in the hippocampus and the entorhinal cortex, episodic memory gets burned away, past and present become forever dissociated, and the patient is at the mercy of cerebral Furies. In this merciless process of dehumanisation, the only means of human communication at the end will be the comforts of touch and possibly some snatches of music.

The story that Jebelli tells of his grandfather’s decline illustrates the tantalising mystery of Alzheimer’s: it’s both highly visible yet agonisingly elusive. Like McCavity, this disease defies all known laws, slipping through the net of neurological inquiry.

The incidence of Alzheimer’s is a lottery. You can live “a decidedly salubrious life”, reports Jebelli, and still get struck down in your 70s, sometimes even sooner, with no obvious cause. In the department of prevention, he takes us through a familiar catalogue of potential risk-factors: stress, diet, exercise, etc. He’s forced to conclude, as every visitor to the catacombs of dementia will eventually acknowledge, that Alzheimer’s “remains an enigma”, a tangle of amyloid plaques, sticky buildups of protein in the brain that continue to resist the investigations of the neurological police.

Here, Jebelli’s own pursuit of answers to his grandfather’s death turns into a fascinating quest at the frontiers of neuro-degeneration. He identifies several key areas of recent research, from cerebral renewal (the implantation of iPS cells) and parabiosis (“reversing the pathological changes in an old animal by bathing its tissue in the blood of a young one”), to the pioneering study of Kuru (a shaking disease found in Papua New Guinea) and the latest research into PCA (posterior cortical atrophy), the variant of Alzheimer’s that afflicted the late Terry Pratchett. In Jebelli’s optimistic summary, “the web of treatment is widening”. At the end of his “pursuit”, he declares: “We are closer than ever to the abolition of Alzheimer’s.”

Not everyone agrees with him, and the dividends of intense neuro-scientific research are painfully modest. From 2000 to 2012, indeed, it’s estimated that about 99% of all newly developed “dementia drugs” failed to pass their clinical trials. For all the tabloid headlines about “a cure for Alzheimer’s”, this goal remains fugitive.

Frustrated by the limitations of neuroscience, some Alzheimer’s experts have begun to argue for an alternative approach. In his Penguin Special on Alzheimer’s, Andrew Lees, an acknowledged expert, focused on a fascinating new genre of Alzheimer’s writing, books by patients at the beginning of their slow fade who can illuminate the experience of losing memory.

Yet even this avenue is contentious. As the Observer reported recently, a new Edinburgh University study, the Prevent Project, suggests that Alzheimer’s may not be the disease of memory that Jebelli describes.

In truth, there has been no shortage of neuroscientific investigations, but it’s hard to resist the conclusion that these have been blind alleys. By contrast, the phenomenology of losing personal cognition (the territory explored by the late Oliver Sacks) offers, from some points of view, a more fruitful cerebral exploration. It might at least give comfort, if not hope.

 • In Pursuit of Memory by Joseph Jebelli is published by John Murray (£20). To order a copy for £17 go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99