Seemingly from out of nowhere, a nasty new virus emerges and sweeps the globe.
Millions become sick and die, and soon enough, the virus has spread so far that humans are stuck with it.
As decades pass, similar strains spring up and spark outbreaks in places as far flung as Russia, the US, Myanmar, Iran and the Maldives.
We don't yet know if this is the future of Covid-19 – but we do know it was the story of A/H1N1, the subtype of the influenza A virus, which killed some 17 to 50 million people a century ago.
Many researchers have turned back to the planet's last major pandemic – the so-called "Spanish flu" catastrophe of 1918-1919 – as a potential analogue for the coronavirus' possible trajectory, if just because it's one of the few road maps that we have.
While the path ahead is shrouded in uncertainty, virologists are generally at least of the view that Sars-CoV-2, and the scourge of a disease that it causes, is here to stay.
"That was the first time this type of influenza came into the population and it's still around today," said Dr David Welch, a computational biologist at the University of Auckland, said of H1N1.
"So, for anyone who thinks that Covid-19 is going to go away, we've got some clear evidence of what's more likely to happen."
As of this week, Sars-CoV-2 had infected at least 233 million people and killed perhaps more than 10 million.
Some 200 countries have registered infections and few, like New Zealand, have largely managed to keep the threat at bay.
Last year, one of New Zealand's staunchest proponents for elimination, Otago University's Professor Michael Baker, told the Herald that it could be technically possible for high-income countries and emerging economies to try to strangle the virus and maintain regional, Covid-free "green zones".
That required three elements: tightly managing borders, effective testing and contact tracing, and an otherwise intense approach to stamping out transmission – a combination demonstrated with draconian might by China, but never managed by US and other western nations.
But even if global eradication was a faint hope then – 2020 also marked the year Africa was declared polio-free – the arrival of variants like Delta and the world's failure to stop the virus all but guaranteed its continued existence.
Now, we are told the coronavirus is probably already "endemic". What does that mean?
"For ecologists, it means restricted to an area - like kiwi are endemic to New Zealand - but to most people referring to Sars-CoV-2, it just means constantly present and not eradicated," said Professor David Hayman, an evolutionary ecologist at Massey University
"To most epidemiologists, however, it means there is a steady incidence of disease."
Hayman, who has spent much of his career studying the lethal Ebola virus disease, had little doubt that Sars-CoV-2 would persist in human populations for the foreseeable future, not just because of its infectiousness, but because many countries had either failed to wipe it out, or had given up trying.
"But it may not become 'endemic' in the epidemiological sense," he added.
"For example, measles persists globally but is not endemic in New Zealand, and seasonal influenza is technically not endemic, but causes annual epidemics here."
So long as there are enough susceptible people in the world – and enough contact between them to maintain spread – it's difficult to say when that shift to endemicity will happen.
An optimistic view among experts is that enough people in the world might gain immune protection against the virus, either from natural infection or vaccination, meaning fewer hospitalisations and deaths.
"If very high numbers of people are immune through immunisation and transmission greatly reduced, Covid-19 may well be a manageable vaccine-preventable disease and be managed like others," Hayman said.
Yet, there'd always be those at risk who couldn't - or wouldn't - be immunised, which meant there'd be an ongoing need for additional testing, case management, and health systems able to withstand the strain.
"And right now, health systems aren't prepared to cope with a highly-infectious disease that creates such a high proportion of cases, which is true of all countries, not just New Zealand," Hayman said.
"The UK might have reduced all restrictions and have a relatively high proportion of the population immunised, but its healthcare system is hugely stressed, with many millions waiting for treatment."
Professor Graham Le Gros, an immunologist who heads the Wellington-based Malaghan Institute, similarly predicted there'd always be pockets of the global population that didn't get infected in any set of years, and therefore couldn't retain immunity.
Babies born in future could eventually lose the immunity passively acquired from their mothers, and also become vulnerable.
In a world crowded with potential hosts and few barriers to transmission, Le Gros said the virus might well maximise its ability to survive by becoming yet more virulent and aggressive, so that it could make people more symptomatic, and thus infect more of us.
On the other hand, if the population was spread out, and we communities tried to counter the virus with health interventions, he said the virus might use a different tactic – producing fewer symptoms so hosts were unaware they were sick and infectious.
And then there was always the possibility that the virus had already found its enduring form: the variant B.1.617.2, better known as Delta.
Despite hopeful commentary from many experts that Sars-CoV-2's destiny is to become a widespread, much less threatening pathogen like influenza, scientists told the Herald this assumption was premature.
That's because variants like Delta have now worryingly shown us that they don't actually need to be milder.
"The virus will only evolve to become less severe if that provides a selective advantage for the virus - and that doesn't appear to be the case," Otago University virologist Dr Jemma Geoghegan explained.
HIV/AIDS, which killed an estimated 680,000 people last year, remained just as virulent today as it was when it first emerged, perhaps 100 years ago.
And if smallpox hadn't been eradicated 40 years ago, Geoghegan said, it would also still be just as severe as it was for centuries.
Welch added that predictions that Covid-19 would essentially become flu-like often tended to be based on models that fell apart when more complexity was applied.
"The main imperative of a virus is to infect as many people as it can, so it mutates to become more infectious: that's the clearest thing you can say, and that's what we've seen in this pandemic."
But there's one element of influenza more likely to be seen in future coronavirus: seasonality.
That wasn't only down to the behaviour of the virus over the colder months, but also our own.
"Because Sars-CoV-2 is transmitted person to person, a lot of the drivers of seasonal infection dynamics are as much to do with human behaviour, such as school holidays, and other complexities," Hayman said.
"So, even if it persisted in a population, there is likely to be some seasonality."
But how much, he added, hinged on a wide range of factors – including levels of immunity.
What difference could vaccines make here?
Newly-released modelling by Te Punaha Matatini researchers suggested that, if New Zealand managed to inoculate 95 per cent of our population over 5 years old, we might only see fewer than 3000 infections, 80 hospital cases and just eight deaths in a year.
These median estimates – which assume that only baseline public health measures and limited test-trace-isolate-quarantine are in place - are far lower than the flu, which annually kills around 500 Kiwis and infects 200,000.
Yet, if our uptake only reached 70 per cent, the country could be looking at 1.7 million infections, around 110,000 hospitalisations, more than 13,000 deaths.
With nearly 18,000 hospital beds required at the peak, the health system would be plunged into chaos.
And at 90 per cent, however, the modelling pointed to 171,000 infections – but only around 6000 hospitalisations, just over 600 deaths, and 438 beds occupied.
But these assumptions also factored in vaccines that had a fair level of effectiveness.
Because no one has been vaccinated for longer than 18 months, it's still unclear how long this immunity may last, although there's some data to suggest that it might wane in elderly people but stay stable in other age groups.
"From what we know about natural infections by coronavirus, the projections are that [the virus] will change over time to escape immunity to previous versions – and also, natural infections do not always stimulate strong immunity," Le Gros said.
A virus that didn't have to contend with lasting, specialised immunity would go on confusing and causing havoc in the human immune system.
While there's been much discussion about Covid-19 booster shots, scientists say there are still too many unknowns to predict whether these will be a semi-regular part of our post-pandemic lives.
Some viruses, like influenza, require repeated vaccination because of its antigenic evolution yet others like measles can be kept at bay for decades after childhood vaccination – and it's unknown where the coronavirus sits on this spectrum.
But Le Gros was upbeat.
Vaccinologists now knew how to stimulate long-lasting immunity to protein antigens like those produced by Sars-CoV-2, he said.
While these were yet to be used in the general population – and mRNA vaccines, like the Pfizer shot we're all receiving, were yet to be modified to pack this ability – Le Gros expected scientists would soon deliver a vaccine with the ideal enduring, sterilising immunity that we needed.
That would make measures like vaccine passports – if not the importance of getting vaccinated – all the more important in a future where we essentially lived alongside Covid-19.
In New Zealand, a vaccine passport being developed by health officials in the form of a smartphone app could be available to download in just over a month's time.
How much longer will Kiwis, and the rest of the world, be enduring the worst of this pandemic?
As tempting as it might be to use 1918-19 as a yardstick and guess two and a half years that, like the three other last global pandemics, were all driven by influenza.
As Le Gros put it: "We are sailing in completely uncharted territory with this virus."
As the year draws to a close, Waikato Herald is taking stock of 2024.