Look at the satellite images below. One shows a hospital car park in the city of Wuhan, China. It was taken in October 2018.
The other shows the same location almost exactly a year later, in October 2019.
Now spot the difference.
That is what a team of researchers from Harvard and Boston Universities are doing.
They have so far pored over 111 images of medical facilities in the place where the coronavirus pandemic began — and those pictures appear to reveal a trend.
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Namely that, from August last year, something was making those medical facilities much busier than normal.
This exercise in "digital epidemiology" was published this week, adding to suspicions that the virus was making people ill far earlier than previously thought.
Officially, China first reported a cluster of cases on New Year's Eve. It later told the World Health Organisation the earliest symptoms for those patients dated back to December 8.
The disease began to spread overseas in mid-January, according to news reports, with cases recorded in the US on the 21st, France on the 24th and Italy on the 31st.
That was also the day two Chinese nationals tested positive in York, making them the first confirmed cases in the UK (though they caught Covid-19 overseas). The first uniquely British case was identified in scout leader Steve Walsh, from Brighton, on February 6. The first confirmed case of transmission in the UK was then registered in Surrey on February 28.
Yet there are growing claims — not limited to the satellite study — the coronavirus may have been in circulation far earlier. And scientists in other countries are turning up growing evidence that Covid-19 appeared on their shores earlier than thought, too.
A summer 'spike' in Wuhan
The "digital epidemiologists" from Harvard and Boston identified an apparent surge in vehicle traffic outside hospitals in the Chinese city that started to become apparent in late August last year.
The traffic spike appears to have gathered pace as the year went on. For example, researchers counted 171 cars parked at Wuhan's Tianyou Hospital one day in October 2018. On the same day in 2019 there were 285 — an increase of 67 per cent.
Things peaked in mid-December, when the local outbreak was at its height, the researchers say. And the growing congestion in hospital car parks in this period coincided with a marked rise in the number of local internet users using search engine Baidu to look up the terms "cough" and "diarrhoea".
In a study published this week, the US team argue that their findings "corroborate the hypothesis that the virus emerged naturally in southern China and was potentially already circulating at the time of the Wuhan cluster" that became public around Christmas.
"Clearly, there was some level of social disruption taking place well before what was previously identified as the start of the novel coronavirus pandemic," author Dr John Brownstein said.
Not everyone agrees, however. The study has not been peer-reviewed and, although the team was able to study 111 satellite images, it was impossible to make comparisons on some days because of cloud cover.
Critics also believe the type of medical facilities depicted in some images may undermine this thesis, as several were children's hospitals. As Keith Neal, emeritus professor of infectious diseases at Nottingham University, points out: "Youngsters simply aren't hospitalised that much by the virus."
But a growing pandemic would be likely to displace patients from elsewhere, increasing strain on all parts of the medical system.
The Chinese Foreign Office has nonetheless declared the study "full of deficiencies" and "grossly fabricated" as part of a US plot to "create and deliberately disseminate disinformation against China".
Athletes keeling over in October
About 10,000 athletes from more than 100 countries descended on Wuhan for nine days last autumn to compete at the Military World Games, a sort of Olympics for serving soldiers.
After the pandemic struck, months later, many recalled how they had fallen ill during the event in October, suffering flu-like symptoms similar to those of the coronavirus.
The Spanish Ministry of Defence has revealed at least four of its delegation had become seriously ill in Wuhan. One of them told El Mundo newspaper: "The authorities just took it as a sore throat or flu infection and treated us as if we were already cured."
Some members of the French team then made similar reports.
Elodie Clouvel, a world champion modern pentathlete, said in a TV interview: "We were in Wuhan at the end of October and afterwards we all fell ill. Valentin [Belaud, her partner, also a pentathlete] missed three days of training. I was sick too ... I think we have already had the coronavirus."
German volleyball player Jacqueline Brock recalled: "Some athletes from my team fell ill. I got sick in the last two days. I have never felt so sick — either it was a very bad cold or Covid-19."
And an Italian fencer, Matteo Tagliariol, gave an interview in which he claimed all six occupants of his official apartment in the city fell ill with symptoms "that looked like those" of the virus.
Catching something that "looks like" Covid-19 isn't the same as actually contracting it, of course. And six weeks after the athletes first made their claims, none appears to have tested positive for antibodies you would expect to find in a genuine survivor.
Cases 'confirmed' after singles day
China's authorities went public about the crisis on New Year's Eve and insist the earliest recorded case dates from December 8.
But secret government data leaked to the South China Morning Post in Hong Kong suggests the outbreak began almost a month earlier.
Documents seen by the newspaper suggest the first confirmed case can be traced back to November 17, less than a week after "Singles Day", a public holiday when young men and women traditionally go out in search of a partner.
"A 55-year-old from Hubei province [the region that contains Wuhan] could have been the first person to have contracted Covid," the newspaper reported in March.
"From that date onwards, one to five new cases were reported each day. By December 15, the total number of infections stood at 27. The first double-digit daily rise was reported on December 17 and by December 20, the total number of confirmed cases had reached 60."
At least 266 people were infected by the end of December, said the report, which stressed that, of the first nine cases identified in November — four men and five women, aged between 39 and 79 — none has been confirmed as being "patient zero". So it is possible that other so-far-unidentified cases date back even farther.
Even if this is true, it doesn't necessarily follow that a November outbreak was covered up. Although the disease may have been doing the rounds at that time, whistle-blowers in the Chinese medical community say doctors only realised they were dealing with a new disease in mid-December.
Suspicious French X-rays
French scientists, on the hunt for their country's so-called "patient zero", have found evidence the coronavirus may have arrived in Europe around the time it started to spread in Hubei.
Dr Michel Schmitt, from Albert Schweitzer Hospital in Colmar, northeastern France, leads a team of researchers examining thousands of chest X-rays taken in late 2019. They have so far been able to identify two scans "consistent" with the symptoms of Covid-19 dating back to mid-November.
The images depict white shadows on the lungs of patients of a type 'typical of" coronavirus.
"The first case was noted in our centre on November 16," Schmitt said in a press release. The other was taken two days later.
Using the same method, the Schmitt team has also identified 12 probable coronavirus cases in December and 16 in January.
The team's findings were announced in May, shortly after another French doctor, Dr Yves Cohen, head of emergency medicine at two hospitals near Paris, revealed he had found a positive test for coronavirus there from December 27.
Cohen's team retested blood samples from 24 patients suspected to have flu. His findings seem to confirm that the virus was in France weeks before it was officially reported there on January 24.
Other European countries are now understood to be examining similar evidence, amid suspicion the outbreak in Italy, in particular, started far earlier than was originally thought.
British expat fell ill in November
Connor Reed, a 25-year-old from Llandudno in North Wales, was the first Briton confirmed to have caught the coronavirus.
The expat teacher, who worked at a school in Wuhan, recalled falling ill on November 25 when he "woke up feeling really bad. I was coughing a lot and lost my voice".
Initially, he put his symptoms down to a standard cold. But over the ensuing days he developed what seemed like severe flu and was forced to stop working by early December. His illness progressed into pneumonia, along with other classic Covid-19 symptoms.
After he had recovered later that month, Reed says his doctors confirmed he was suffering from coronavirus. In theory, that means he fell ill long before China's authorities accept the outbreak started. But it is possible he caught a cold two weeks before contracting the virus.
Another Briton who may have been infected in Wuhan at around the same time was Andy Gill, lead guitarist for the band Gang Of Four, who died in February after visiting the city during a November tour of Asia.
His widow, author Catherine Mayer, has claimed in a blog entry that Gill was admitted to hospital in the UK in January after developing a "respiratory illness", with the cause of death officially listed as pneumonia and organ failure.
Writing on her website, Mayer revealed Gill had suffered several of the symptoms of Covid-19, including low oxygen, lethargy and diminished appetite. One of Gill's doctors said there was "a real possibility that Andy had been infected by Sars-CoV-2".
Although tests came back negative, this was not, the specialist explained, a definitive answer. "By the time of Andy's admission to hospital, he had been ill for weeks," she wrote. "The virus could have already left his body but triggered immune complications."
December — US victim in hospital
The first US case of Covid-19 was announced on January 21 in Snohomish County, just north of Seattle, when a man who had recently travelled to Wuhan fell ill.
However, it recently emerged that two other locals, who became ill in December, had also tested positive for Covid-19 antibodies, suggesting the disease may have arrived in America a month earlier than previously though.
One victim, a woman identified only by her middle name, Jean, told the Seattle Times she became unwell just after Christmas with a dry cough, fever and body aches. She made two trips to the doctor and was sufficiently unwell to be given chest X-rays and a prescription for an inhaled medication.
At the time, Covid-19 had not been formally discovered or named, so she was not tested. Her more recent positive antibody test result is not definitive proof she had Covid-19 in December because it doesn't show when she was infected. But officials say a second local who reported similar symptoms at the time has also tested positive.
"The amount of air travel into and out of Wuhan was enormous, probably thousands and thousands of people," Dr Art Reingold, a public health epidemiologist at the University of California at Berkeley, told reporters. "It follows that there [were] likely multiple introductions around the world, quite possibly in December."
UK skier 'infected on New Year trip'
The availability of antibody tests is now providing growing evidence that Covid-19 entered the UK shortly after Christmas. This week, a London art director who fell ill on January 6 after returning from a skiing holiday in Austria was given a positive result.
Susannah Ford, 53, said she is convinced she caught it at the Obergurgl resort, which is less than eight miles from the Italian border and 65 miles from Ischgl, which is facing a criminal probe over allegations that it covered up an outbreak for weeks.
"I felt like death," she told The Sunday Times. "I ached terribly in every muscle and joint for five days and was too groggy even to go to the Eliot Prize for poetry."
Her test result, of course, does not indicate when she caught the disease, so she could have contracted it later (despite not having suffered any symptoms).
Much of this apparent supporting evidence is anecdotal. Many Britons think they suffered from Covid-19 around Christmas, at the time believing it to be just a very bad flu. But apart from Susannah Ford, few of them have so far tested positive for antibodies.
Professor Sebastian Johnston, an expert in respiratory medicine at Imperial College London, whose company Virtus Respiratory Research has developed a highly sensitive quantitative Covid-19 antibody test, told the Mail he has so far used it on about a dozen people who "had symptoms consistent with Covid from January and back through to December, and had got the idea that they may have had the virus".
Not one has come back positive.
Johnston does not believe the coronavirus was doing the rounds in the UK much earlier than official data shows. He says: "The first death is a sign that it has reached pretty widespread community transmission, and in the UK ours was not until March 5."
The date of the first confirmed case in the UK remains January 31, and transmission was not recorded here until the end of February.