I have written about how the disease presents in a person, my experience of caring for my mother, the causes, my quest to find a cure or treatment that might slow it, and the lifestyle modifications we can make to protect our brains.

Alzheimer’s and other dementias don’t suddenly descend in age, says James Rowe, Professor of Cognitive Neurology at Cambridge. “They develop, hidden, over decades, like a slowly brewing storm.” From midlife, that pathology accelerates “and eventually breaks cover, overwhelming the brain’s ability to compensate.”

Brennan introduces her book with a poem which explains the title and reminds us of the central themes of anybody’s dementia journey:  I have dementia and I am still me, and I care for you and I am still me.

Photographs of my mother, hers, me cradling my newborn son against me (“Look! Four generations,” we’d laughed that day). Mum carefully considers this little group, standing smiling on the back step of a home bathed in September sunshine: “There’s my mum!” she said.

Then, pointing at me: “Who’s she?”

Mum’s depressions started in her thirties. How much of a role did they play in the dementia that revealed itself in her seventies? There are several compelling hypotheses that suggest one drives the other.

A study published this week suggests that statins might be the latest to join the arsenal against Alzheimer’s. It’s a tablet millions of people pop daily, my husband among them. My mother, who died of dementia, also took statins — but only after having had a stroke in her seventies. Is it possible that had she taken them earlier in her life, she would have been spared from the development of this cruel disease?

The author enlists her investigative journalism skills to understand her mother’s illness and, later, to learn how she might save herself from the same neurodegenerative fate.

Dementia may feel like a wrecking ball, but it’s not simply absent one day and fully present the next. No, it settles in, makes itself at home and lingers in the shadows for 10, 15, or even 20 years.

I watched my mother draw a clock while she was in a brain rehabilitation centre after her stroke. Until then, I did not know how common a test it was, how often it was used – or why.

The clock drawing test – or CDT – is used regularly to assess several mental processes. Over the past two decades, it has become a key tool in early screening for cognitive impairment – especially dementia.

You might expect it to be simple: drawing a clock that shows the time, say, at six o’clock: a circle, numbers, a big hand, a little hand.

It was not hard to imagine how I might have fared in my mother’s shoes. I would have battled to pick up a mug of tea and navigate it to my mouth without spilling. Experiencing the same effect as headphones playing senseless sounds in my ears, I would not have been able to focus or think straight.

And then I remembered that into my mother’s confusing, frustrating world and through the non-stop hiss and rasp of unwelcome sounds and disembodied voices, there was I – her carer – insisting it was time for a shower or that she must finish her juice or go to the loo.

Did my words get through to her? Or did they just add to the upsetting noises in her head?

Breakfast was once eaten enthusiastically. Mum declared everything delicious, especially the marmalade she ate every day, but professed never to have eaten in her life. Near the end breakfast was a miserable exercise in spoon-feeding. Using the telly as distraction I might force another mouthful in.

Dementia, I discovered, isn’t just common — the UK’s biggest killer for a second year running — and it’s about more than lost memories. It stole Mum’s bearings so she couldn’t orientate herself (“Where am I?”), her family (“Who are you?”), her mobility (“What do I do with my feet?” when I urged her to walk) and, in the end, it stole her.

Sometimes dementia is a loaded gun; you’re going to get the single bullet in a chamber anyway, whatever you do: Russian Roulette. Sometimes it isn’t. The only thing that I know for certain now is that it is not inevitable with age.

People asked me afterwards, a little incredulously: “Was it really so sudden?” Yes. One day, late in 2019, between lunch and tea, my mother forgot me. I sat across a table from her at one meal and she was entirely confident who I was. That evening: “Tell me”, she asked, “When did we first meet?”

OCD is complex and commonly misunderstood, with a limited number of treatment options. But in recent years, the mechanisms in the brain and body that drive it are finally being pinned down, revealing an elaborate picture involving genetics, various brain networks, the immune system and even the bacteria in our gut. In turn, this growing understanding is opening up new possibilities of tackling this life-sabotaging condition.

The evening my mother forgot who I was — who I am — was just like the one that had come before, and the one before that. Weeks of evenings all alike. I puzzled about that later. Why that evening? Why so sudden? So that at lunchtime she knew I was her daughter and by nightfall she didn’t. Six hours later. That’s all it took.

The Tsimané are an indigenous people from the Bolivian Amazon in South America. You have probably never heard of them. I had not. I had to look up how to pronounce Tsimané: chee-mah-nay.

And yet this small group is extraordinary. A few years ago, scientists studied them to understand their exceptional heart health: 85 per cent had almost no evidence of the calcification of the arteries that is the marker for atherosclerosis. Even those who did have some had very little.

Depression, says Professor Craig Ritchie, the chief executive and founder of Scottish Brain Sciences, “may well be an upstream trigger for physical health”. It might even have been a significant risk factor for the Alzheimer’s disease that my mother suffered from in the last six years of her life.

People may present with depression later in life as a consequence of dementia but increasingly, research points to depression in early and midlife as a risk factor for developing dementia.

In a photograph of my mother on my desk, she is smiling broadly, an even, white-toothed smile. It was taken 18 months before she died. Her dementia was evident everywhere in our lives by then – but not in that picture: from the photo you’d never guess. She looks self-possessed and whole. In fact, she looks like a commercial for geriatric dental care with that wide, white smile.