There’s a silent crisis in our midst. Often cunning and stealthy like Grandmother’s Footsteps, sometimes swift and brutal, dementia is all around us. One in eight people die of it. In the UK, a person is diagnosed with it every three minutes. Yet we are still collectively failing to acknowledge this self-demolishing illness, which is now the largest cause of death in England and Wales and the one we most fear. Why?

I’ve recently been doing talks around the country as part of John’s Campaign (named after my late father, who lived for 10 years with the disease), which I co-founded to encourage the NHS to collaborate more with families in the care of dementia patients. There is always time given to questions from the audience at the end. Often these turn out not to be questions at all, but confessions, or acts of bearing witness, or simply pleas to be heard, understood, recognised. In a hall in London, a young woman puts up her hand, stands, opens her mouth, then instead of asking a question she bursts into tears. In a church in the north of England, an old woman speaks of how for 15 years she cared for her husband with a particularly savage form of early-onset dementia until he died and how she was now the sole carer for her son, who has the same illness. She speaks with a terrifying calmness; it’s only when those around her offer help that her face crumples in grief. A man with white hair asks, “What shall I do? What?” A daughter asks, “How do we tell him what is wrong with him?” A son asks, “How long can it last?” He also means, “How long can I bear it?”

Dementia is a terminal illness; there is no cure yet, only care. It usually comes gradually, a radically slow-motion form of dying in which the patient is increasingly at the mercy of those who look after them. Along with its particular medical symptoms, it can gather up fear, guilt, shame, loneliness, and a desolation that spreads out from the person who lives and dies with it to the people who love them. Its costs are enormous – not just the staggering financial cost to the individual and their family and to society, but the psychological and emotional ones.

We turn away from dementia because we fear it and feel helpless in the face of its advance. It is not simply that it is so common (about 900,000 people in the UK have the diagnosis), but that it uniquely threatens our identity and reminds us of our vulnerability. We spend our lives building up our defences and learning how to be a social being: we are continent; we keep some thoughts to ourselves; we have secrets and compartmentalised lives; we tell the necessary lies and perform the self that we want to be.

All of this control gradually (or sometimes not so gradually) unravels during the dementia years, until at the end the boundaries are breached so that the self pours unmediated out into the world, the world flows into the self. It can be terrifying to see a person in the end stages of dementia because they show us the chaos and shattered meanings that always lie just beneath our own constantly patrolled surface. We know they could be us one day; we push away that knowledge.

We live in a society that values youth, success, health, vigour, self-sufficiency and purpose. So what about the person who is old, frail, confused, increasingly dependent on others? And what about ourselves when we come to that state, as most of us will, whether we have a diagnosis of dementia or not? Perhaps we fail to properly acknowledge dementia because to do so would be to acknowledge our own vulnerability. The frequently demeaning terms for the man or woman who has advanced dementia (they are bed blockers; they have lost their marbles; lost the plot; they are vegetables; the old dears; old fools; bodies in a bed; burdens on the state; are no longer “us” but “them”) reduce them to objects, not subjects of their own lives. As objects, we can ignore them, and ignore our future disgrace. In making them less than human, we make ourselves so as well.

Dementia does not just happen to old people – around 5% of people with Alzheimer’s are under 65 – but it usually does. It is easier to turn our faces away from the suffering of the old; even the old don’t feel themselves old. Moreover, people who have dementia are quite often unable to speak for themselves. They, and often those who care for them, become missing people. The illness progresses out of sight: in kitchens and bedrooms, in residential homes on the edges of towns, in hospital wards, on the fringes of our collective consciousness.

Dying is what we all do, but we need to do it better. The diagnosis of dementia need not be a terrible sentence, but the beginning of a new chapter. People can live well, happily and adventurously with it for years and even decades. They can also live in loneliness, loss and desolation. One day there may be a cure for the illness – or many cures, because there are many different forms of dementia. For now, there is only care. Across the country, in thousands of homes, men and women are invisibly performing this most difficult act of care with scant support. They often labour alone, feeling that it is an individual piece of unfairness and bad luck, one of life’s ambushes.

Dementia should be and must be our collective responsibility. A good place to start is in collectively facing up to the fact that it is in our midst and that each year hundreds of thousands of men and women are living with it and dying with it. If not you, someone very near you. If not now, soon.

• Nicci Gerrard is the author of What Dementia Teaches Us About Love