How do you kill a virus? How do you eliminate a miniscule genetic strip of instructions whose purpose is to replicate itself?
On our skin, of course, soap is highly effective. It works by washing its way through the layer of fat that encases coronaviruses so that, as Dr Charlotte Houldcroft, of the University of Cambridge, vividly puts it: "The genetic guts of the virus spill out."
Inside our bodies, the task is much harder. So we wait for drugs to treat the worst effects of SARS-Cov-2, and vaccines to prevent it from getting a hold in the first place.
But there is another way, an endgame for coronaviruses even without treatments and vaccines. Already this century, devastating outbreaks of deadly cousins of today's virus have twice been crushed without global immunisation programmes. So, as countries around the world begin to relax their lockdowns, will the third time be lucky too?
"There's no reason we have to have a second wave," says David Heymann, Professor of Infectious Disease Epidemiology at the London School of Hygiene and Tropical Medicine.
Few people are better placed to know. During a two-decade career at the World Health Organisation, Prof Heymann headed the global response to Sars, the disease which emerged in 2002 from another coronavirus (SARS-Cov-1) and was so strikingly similar to today's that the current incarnation is named after it.
In 2002, that virus also made the species jump at a market in China late in the year, in November or December. Then, too, the outlook seemed grim. "It was spreading quite easily in some communities," says Prof Heymann. Whereas the infection fatality rate (IFR) for Covid-19, the disease caused by today's coronavirus, is thought to be less than on per cent, Sars killed one in 10 of those who caught it.
But then, a little more than six months after it had emerged, the Sars virus was eliminated. The last cases, excluding accidental release from labs, came in July 2003. Now, says Prof Heymann, we are once again at the six-month mark. "It will be an important thing to take stock in July, to see where this virus is," he adds.
There are, after all, three fates for any new virus that manages to cross the species barrier and infect humans. The first, from the point of view of the virus, is a dead end. Like rabies, it will infect one person and go no further.
The second fate awaits those which cause outbreaks but then, with more or less difficulty, can be ushered back from whence they came. Ebola, thought to jump to humans from bats or monkeys, is the classic example.
The third scenario is where the disease cannot be eradicated and becomes, instead, endemic. Think HIV.
"We don't yet know the destiny of this virus," says Prof Heymann. "It could become endemic in humans. It could mutate in such a way it becomes more or less virulent. Or more or less transmissible. Or it could disappear." Sars, in the July following its emergence, disappeared.
That disappearance, it is important to note, did not happen on its own. Rather, it was the result of the first global public health response which, thanks to modern communications, unfolded in more or less real time. Then, too, there were travel bans. Then, too, there was tracing and co-operation across borders between researchers.
This time around, says Prof Heymann, such co-ordination has hit problems. "The world is working together at the technical level and the scientific level, but not so well at the political level… [Covid] has become involved in geopolitical accusations."
Sars eventually infected around 8,000 people, with 774 deaths. At the WHO, this toll was considered a narrow escape. "We were almost ecstatic," recalls Heymann.
In the event, SARS-Cov-1 proved something of an ally to the efforts to corner it, causing people to spread the disease only when they were very sick. Many infections occurred in hospitals. Tracing was, as a result, far easier than it is today.
Even so, there is another factor – a great unknown. Many viruses become less virulent over time in a process known as "attenuation". It happens to influenza viruses, and to others like the Monkeypox virus that can also jump the species barrier to humans. "Many times," says Heymann, "viruses eventually cause less serious illness with passage through humans, we assume through mutation."
SARS-Cov-2 might simply fade away.
Mutation is certainly occurring to SARS-Cov-2. At Cambridge, Houldcroft is part of a team leading a £20 million initiative among institutions across the country known as COG-UK. It is tracking genetic mistakes, which sometimes occur as the virus replicates and so add new branches to its family tree.
Most mutations are trivial. But some are significant enough to become different strains of the disease that may be more, or less, dangerous than the original.
A recent paper by Manuel Becerra-Flores and Timothy Cardozo at New York University, for example, suggests that many European patients may have a strain slightly deadlier than that of Asian patients, and that these strains may have seeded outbreaks on the east and west coasts of the US respectively, perhaps explaining the higher toll in America's cities on the Atlantic than those on the Pacific.
It was genetic tracking that ultimately proved that Mers, this century's other – particularly lethal – coronavirus (with an IFR of more than one in three), which emerged in Saudi Arabia in 2012, was unlikely to cause a pandemic. Once it had jumped from camel to human, it struggled to infect other people. "The transmission chains died out," says virologist Dr Gytis Dudas. "Mers is sustained in camels, and humans are a dead-end host."
But in today's pandemic, humans are clearly not a dead-end host. How, then – short of some benign mutation or a successful vaccine – might SARS-Cov-2 meet its end? In all probability, the only way will be to smother it, starving it of new hosts in which to replicate.
In the UK, there is a long way to go. Confirmed new cases are running at under 2,000 per day, but the ONS suggests the likely true number is around 8,000. In China (eight new cases per day) and South Korea (46) the picture, if we accept official figures, is very different; and in Taiwan and Vietnam the contagion appears to have been almost completely eliminated.
What unites these countries is that they were the worst afflicted by the Sars outbreak. South Korea was badly hit by both Sars and Mers. "The Asian countries which responded to Sars and Mers were rapidly on top of this from January," says Prof Heymann.
"They immediately began contact tracing, isolating and controlling the outbreak. And they're using [those tactics] now instead of the general lockdown procedure. It's an entirely different approach based on epidemiology."
Contact tracing and social distancing, he says, are the critical circuit breakers, disrupting transmission.
No need to sing about the latter measure: choirs show that crowded rooms with spit flying about are this virus's ideal habitat. One group in the US suffered an 87 per cent infection rate after a rehearsal in March. Packed dorm rooms for migrant workers also prove the point, and have today made Singapore the exception to the protective Sars legacy in Asia.
Canada is the other outlier among Sars veterans. It was hit badly in 2003, with 43 deaths. Yet today it is still suffering more than 800 new coronavirus infections a day. According to a report in The Lancet, while many improvements were made to Canada's health system after 2003, the "epidemic surveillance system" to track outbreaks – demanded by a post-Sars inquiry – was not among them.
This year, then, the world fell into two categories: those whose experience of Sars told them transmission could be interrupted by tracing and local quarantine; and those with no experience of coronaviruses, using influenza models, which didn't understand that transmission could be interrupted and so, according to Prof Heymann, were saying: "The only way to deal with this is mitigation."
It is the speed of nations' switch from the latter mindset to the former which has had perhaps the most telling effect on their situation now.
Sars did not end by some miracle in July 2003, says Prof Heymann. It ended because affected countries followed a time-consuming playbook: "Actively identifying cases, isolating them and then doing contact tracing and making sure that the contacts were self-quarantined if they became sick, were tested and isolated themselves."
This year, he says, Germany "continued that in Europe, as did Switzerland and some of the other countries that are now way ahead. But Asia did it from the very beginning."
Such tracing is incredibly demanding, but effective. Some 40 days into their respective Covid-19 epidemics, Germany had more new cases than Britain. Today, there are just 430 new cases per day among its 83 million population.
It also took 40 days, and an army of tracers, for Germany to get from the level of the UK's current new caseload (2,000 per day or so) down to that figure of 430. And South Korea shows that daily infection rates can fall from 450 or so to single figures in 47 days.
There are caveats: Germany's greater testing means its official infection figures are almost certainly a better reflection of reality than those in the UK. And South Korea has seen a small spike of cases since it managed to get its new infections down to fewer than 10 per day.
But these countries show that it is possible, without lockdown but with effective tracing, to get from where we are now (at least officially) to near extinction of the virus in under three months. All without a vaccine.
And if that were to happen, not just in the UK but in countries around the world, SARS-Cov-2 would find itself with few places to go. "You would wonder then whether there are enough people that the virus can transmit," says Prof Heymann. "That would be wonderful."