“There are decades where nothing happens; and there are weeks where decades happen.”

So said Vladimir Ilyich Lenin of the ferment of revolution, but he could just as easily have been talking about the 100 days that have passed since the moment coronavirus officially became a global phenomenon, the day China reported the new contagion to the World Health Organization.

The world has been transformed in that time, perhaps nowhere more so than Britain.

A hundred days ago, on 31 December, the UK prime minister delivered a video message full of hope and promise.

The coming year would, he said, be a “fantastic” one, the start of “an exhilarating decade of growth, prosperity and opportunity”. In 2020, he enthused, Britain would brim with “confidence”.

The early weeks suggested the PM might be right on one count at least. After three and a half years of rancour over Brexit, some of the poison began to drain out of the issue. Of course, it wasn’t “done”, as Johnson promised it would be, but it seemed as if we might dwell on lesser worries.

We saw in 2020 debating Megxit, a country with no greater angst on its mind than whether the Sussexes should carry on royalling.

On 31 January, the UK formally left the European Union. This new coronavirus was low down on the bulletins, safely tagged as foreign news.

Even by early March, it had not quite bared its teeth. People knew the official advice but weren’t sure quite how seriously they were meant to take it. Those politicians involved in public health messaging might attempt an awkward elbow bump at the start of a meeting, only to end it with a handshake or even a bear hug.

Johnson himself, at a press conference on 3 March, cheerfully boasted that he was still shaking hands with people he met – including, he said, people infected with coronavirus.

And yet, after a couple of those weeks in which decades happen, on 23 March Johnson was delivering a TV address to the nation, announcing a lockdown in what might have been a hackneyed scene from dystopian fiction. The pubs were closed, along with the football grounds and the cinemas and the theatres and the schools. Places that normally throb with noise were suddenly quiet and have remained so.

You can jog through Leicester Square, London, a place normally teeming with tourists, and hear nothing more than the flapping of a distant flag.

Two weeks on from that original edict and now the death toll is in the thousands with the prime minister himself in intensive care, a development that shook people who did not expect to be shaken. Decades, in weeks.

This is a story of change so rapid, we can barely absorb it.

People focus on the questions that are human scale and therefore digestible – how long is the queue outside the supermarket? Do I need to wash vegetables if they’re wrapped in plastic? Can I walk in a park if everyone else is walking in the same park? – perhaps because the larger questions are too big to take in, including the largest of all: is this plague going to kill someone I love? Will it kill me?

This is the greatest UK public health crisis in a century. It threatens a death toll in five figures. It dwarfs any such menace since the Spanish flu afflicted a nation already staggering from the losses of the first world war. Perhaps it will come to seem like an act of God that none of us could have done anything about, a plague on all our houses that could not be averted.

Or maybe a future public inquiry will examine the fact that doctors and nurses were denied basic protective equipment, that care workers were forced to use bin liners for aprons and Marigolds for gloves, along with the paucity of ventilators and, above all, Britain’s apparent inability to follow the WHO’s instruction to “test, test, test”, and conclude that the UK response to Covid-19 ranks as one of the severest failures of public administration in the country’s long history.

That makes this a political crisis.

“They were very slow. They didn’t understand the scale of this,” says one senior figure, who has witnessed the government’s response close up. He says those at the top were “blase”, that emergency Cobra meetings were nothing like the efficient coordination exercises that have followed terror attacks, but “chaotic”, lacking “decisiveness”.

As for the PM, “I was surprised at how not in control Johnson appeared to be.” There was a lack of comparative data on how other countries were responding, a lack of thinking strategically or several moves ahead. Put simply, he says, the government was “winging it”.

The cabinet has looked callow in this period, lacking the seasoned faces of cabinets past. Dominic Raab, Rishi Sunak, Matt Hancock: they don’t have that many years on the clock.

Every time a Michael Heseltine or Gordon Brown comes on the radio, social media brims with nostalgia for the heavyweights of yore.

It’s one reason why the weekend just gone seemed to calm nerves. On Saturday, Labour elected a new leader who looked competent and capable. That brought one sigh of relief. Sunday brought another, as the country heard from its longest-serving public figure, its head of state.

The Queen’s ability to reassure rests on her status as monarch, of course, but also on her extraordinary longevity at the centre of our national life. As she reminded viewers of her TV address that night – a vanishingly rare event in itself – she has been communicating with Britons at moments of distress for an astonishing 80 years.

She recalled broadcasting to child evacuees in 1940, thereby summoning up the mystic power of the event which serves as the foundation story of modern Britain – the moment when we stood alone against an evil menace, and prevailed. Her promise that “we will meet again”, at once a glance back to the wartime past and a glimpse of a more hopeful future, will be remembered as one of the most significant – because necessary – acts of her 68-year reign.

Had the weekend ended that way, a calm might have settled on the land. As one observer noticed, the Queen’s message, along with Starmer’s election, suggested the scaffolding of the British state was being hoisted back into place.

But the calm lasted less than an hour, the nerves jangling once more with the news that the PM had been taken to hospital – proof that even the most protected individual in the country, a Falstaffian figure of hale and hearty vigour, was not beyond the claw of this dreaded virus.

Even so, despite the fear and the loneliness and the claustrophobia and the economic hardship of lockdown, few would say the country has sunk into despair.

Privately, our lives have been pared down to their barest essentials: no sport, no live entertainment, no nights out – just work, for those who still have it, family and remote contact with friends.

The work has changed – all laptops, pyjamas and Zoom for those who once toiled in offices – while family life has changed too, becoming much more concentrated and intense.

For some, that has been an unexpected joy; for others, it has been suffocating and even dangerous.

But our public life has also been stripped to its essentials. We’ve come to see what’s indispensable and what is not.

It turns out that we can function without celebrities or star athletes, but we really cannot function without nurses, doctors, care workers, delivery drivers, the stackers of supermarket shelves or, perhaps unexpectedly, good neighbours.

If you didn’t value those people before – some of those belatedly recognised as key workers are among the lowest paid – you surely value them now. In a new tradition, we emerge from our homes and start clapping every Thursday night at 8pm to make sure they know.

Almost everything the prime minister predicted a hundred days ago has failed to come true: 2020 will not be a year of growth or prosperity, but the very opposite. And yet, on one thing he was right. Somehow, we have left the widest rift of recent years behind.

Leave or remain now feels like an ancient divide, made suddenly irrelevant when the only distinction that matters is alive or dead.