It is the mystery new disease that has put authorities around the world on high alert. Already, nine people have died and around 500 others have been infected after contracting a never-before-seen pneumonia.
The disease, which emerged in China, has also crossed borders with cases in the United States, Thailand, South Korea, Taiwan, Macau, Japan and the Philippines. Experts have warned there is a "possibility" it could also reach the UK and Europe.
Tests have confirmed the presence of a novel coronavirus one of a family of viruses which have proved particularly deadly over the last 20 years, infecting more than 10,000 people and killing around 1,600.
Experts are now asking if this new disease has similar epidemic potential.
But with so much still unknown, the authorities will have a battle on their hands to contain it.
What do we know about the disease so far?
Details are still sketchy. Early cases centred around a seafood market in the city of Wuhan in central China. On Monday, China reported that the virus had spread across the country, including to Beijing, raising concerns as millions begin trips for the Lunar New Year.
A day after state authorities said the virus was "controllable", officials said a third person was confirmed to have died and there were nearly 140 new cases.
In Wuhan, the central city where the coronavirus was first discovered, 136 new cases were found over the weekend, the local health commission said, without giving details about the third death.
The disease has already crossed China’s borders, with two cases in Thailand and another in Japan. All patients had travelled from Wuhan.
A total of 201 people have now been diagnosed with the virus in China. In Wuhan, 170 people are still being treated at hospital, including nine in critical condition, the city health commission said. Health authorities in Beijing's Daxing district said two people who had travelled to Wuhan were treated for pneumonia linked to the virus and are in a stable condition.
Wuhan is a city of 11 million inhabitants and is a major transport hub, including during the annual Lunar New Year holiday when hundreds of millions of Chinese people travel across the country to visit family.
Coronaviruses are common in animals but so far only seven, including this latest one, have ever made the jump to humans.
Most of the coronaviruses that have spilled over into humans cause mild symptoms – much like the common cold – but two, severe acute respiratory syndrome (Sars) and Middle East respiratory syndrome (Mers), have proved particularly deadly.
What don’t we know about the disease?
A lot! Like all the devastating disease outbreaks of recent years, including HIV and Ebola, coronaviruses emerge from animals – in the case of Sars the disease jumped from bats to civet cats, a delicacy in parts of China, to humans. And in Mers the disease leaped from bats to camels to humans.
Now, authorities are scrambling to uncover the animal host of this new virus - a vital piece of the jigsaw.
Professor Ian MacKay, a virologist at the University of Queensland in Australia, said fish or seafood are unlikely to be the culprit. The market at the epicentre of the outbreak also traded in poultry, marmots and bats.
“We don't think of seafood when we think of coronaviruses so the other animals at the market might be a more likely source of the human cases.
“But this is all up in the air. We should always keep an open mind as the investigations that go into each outbreak teach us new things,” he added.
Here we show how coronavirus mutates from an everyday animal virus to a potentially deadly human pathogen:
The other big question now is whether the disease can spread from human to human - and the first evidence of transmission between close contacts is beginning to emerge.
A husband who worked at the market and his wife, who did not, have both fallen ill with the disease. And the patient recovering from pneumonia in Japan told investigators he did not visit any markets in Wuhan but did have close contact with a pneumonia sufferer.
The World Health Organization currently believes that spread between close contacts is possible - but nothing further.
“From the information that we have it is possible that there is limited human-to-human transmission, potentially among families, but it is very clear right now that we have no sustained human-to-human transmission,” said Maria Van Kerkhove, acting head of WHO's emerging diseases unit.
One clear sign that the virus is not highly contagious is that no health workers – who are usually the most vulnerable in emerging disease outbreaks – have fallen ill.
Professor David Heymann, an infectious disease expert at the London School of Hygiene and Tropical Medicine (LSHTM) and the person who led the global response to the Sars outbreak in 2002 to 2003, said: “This is a good sign because it means there is good infection prevention and control in hospitals - but it also means that the virus does not spread easily.”
However, the fact that new cases have emerged in recent days is a worry, said Jimmy Whitworth, professor of international public health at the LSHTM.
“[The Thai patient] sounds like somebody who has slipped through the net. If one person has, have others?
“It doesn’t seem to be a virus that’s rapidly and easily transmitted in the human population, though this does raise the stakes slightly,” he added.
Researchers at Imperial College, London believe the official figure is a gross underestimate and they believe that there maybe more than 1,700 and perhaps as many as 4,000 in a worst-case scenario.
They base their calculation on the number of people travelling in and out of Wuhan, the incubation period between contracting the virus and onset of symptoms and the catchment area of Wuhan airport.
They wrote: "It is likely that the Wuhan outbreak of a novel coronavirus has caused substantially more cases of moderate or severe respiratory illness than currently reported. The estimates presented here suggest surveillance should be expanded to include all hospitalised cases of pneumonia or severe respiratory disease in the Wuhan area and other well-connected Chinese cities."
What are the symptoms of the disease?
In the early stages of the disease the symptoms are fever, fatigue and dry cough. As the illness progresses, patients may experience difficulty breathing.
Older patients with underlying disease seem to be more likely to have severe illness - this is certainly the case in the two patients that died.
But these symptoms - similar to many other respiratory diseases - are what make it so difficult to control.
The virus can be spread through coughing or when someone touches a contaminated surface such as a door handle. And in the early days of a new disease health workers may not know they need to take special precautions, such as wearing masks, to minimise the spread.
This was a particular risk with Mers which cut a swathe through health facilities - the most dangerous point was when infected patients arrived in the emergency department undiagnosed and could spread disease to health staff and other patients.
Is there any treatment or vaccine?
As with other coronaviruses - including the common cold - there are no specific drugs, and treatment focuses on relieving symptoms.
There is no vaccine but various groups around the world are working on a Mers vaccine which could be used as a platform for this new virus, if it develops into a bigger epidemic.
How does this compare to Sars and Mers?
The key difference between the new virus and Sars or Mers is that there is no strong evidence of human-to-human transmission.
However, at this early stage it is hard to predict what the virus will do, said Ralph Baric, professor of microbiology and immunology at the University of North Carolina.
“The biology of a virus is somewhat unpredictable when it first comes out of an animal. It’s at its most vulnerable - it’s found a new host and is trying to get transmitted so it can continue to grow and replicate.
“It’s having to overcome barriers it’s never seen before and it’s a matter of evolution and random chance - how close people are to each other, the air temperature. So many factors are in play,” he added.
Before Sars emerged in 2002 it was widely thought that coronaviruses only caused mild illness and could not kill on a large scale, said Prof Heymann.
“But every once in a while the virus somehow mutates or changes and becomes more uncharacteristic. This is a random event that no-one can predict,” he said.
Sars was first reported in China in November 2002 - by March 2003 it had been found in Thailand, Singapore, Hong Kong, the Philippines, the United States, Germany and Vietnam. It eventually spread to 27 countries in total, infecting around 8,000 people and killing 700.
It was an explosive outbreak, spreading quickly, but then appeared to run itself out - although health authorities are on the alert for its re-emergence.
Mers on the other hand has proved more tenacious - it first emerged in 2012 in Jordan, when it jumped from camels to humans, and then spread throughout the Arabian peninsula.
Around 2,500 cases of the disease have been identified so far and, while the disease hit a peak of more than 600 cases in 2014, there were still more than 190 cases last year. It is more deadly than SARS, and has claimed around 850 lives in total.
This map shows the global impact:
An interesting feature of Sars was that the majority of people who became infected had symptoms, sought medical help and therefore the disease was able to be contained and controlled, said Prof Baric. There was also a cull of the animal source.
Mers on the other hand can spread throughout a healthy population who may not necessarily become ill but who can pass it to other susceptible individuals.
In Abu Dhabi, where there have been 89 cases of the disease since 2012, tests on camel workers showed that many had Mers antibodies but suffered no symptoms. It is thought they have built up a natural immunity to the disease.
Instead, said Dr Farida Al Hosani, a communicable diseases expert at Abu Dhabi’s department of health, most of those who became ill had just occasional contact with camels or camel workers. They were predominantly older people and with underlying conditions such as diabetes or cardiovascular disease.
“Super spreaders” can also emerge - one patient with Mers travelled from the Middle East to South Korea, resulting in 186 infections and 38 deaths.
And a Sars "super spreader" was a doctor, who had treated patients with the disease on mainland China and then travelled to Hong Kong for a family gathering. It is thought that around 80 per cent of all the cases of the disease in Hong Kong can be traced back to this one patient.
He also passed the disease to international travellers staying in the same hotel who then carried the disease back home.
Why do these diseases often emerge in China?
China is a populous country with a large number of animal markets - people are far more likely to come into close contact with live animals that harbour viruses than in the UK.
Another factor worrying experts is the upcoming Chinese lunar new year holiday which will see a huge movement of people around the country with the potential to spread illnesses far and wide.
“With Chinese new year and millions of people travelling, even with limited transmission there’s some chance that this could transmit further,” Prof Whitworth said.
Do I need to be worried?
In a word “no”. Public Health England has put no restrictions on travel to China, or even Wuhan.
And if cases were to come to the UK the strong NHS surveillance system should enable rapid diagnosis, said Prof Heymann. Quick identification of the illness will also be aided by the fact Chinese scientists have already shared the genetic data of the virus online.
“When Mers came into the UK the genetic sequence of the virus had already been shared on the web and the UK was able to confirm it was the same sequence very quickly,” he said.
In 2002 the Chinese were criticised for covering up the outbreak - their approach is more open this time, said Prof Mackay.
“The pace of their discovery and the open reporting of their findings to the world has been phenomenal to this point and they are to be applauded for that.”
But there are still many unknowns - is the virus travelling through the population at a low level, waiting to mutate into a more easily spreading virus? Are there more cases, as the Imperial researchers believe? And what is the animal host?
Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, said this is crucial to containing the outbreak.
“We really need more information about the likely animal reservoir and what testing has been done in the market. The same food going to that market could be going elsewhere.
“So - before we assume unrecognised person-to-person transmission is to blame for the spread - it could be that the animals which originally caused the outbreak are still coming into the food supply and bringing more outbreaks in the Wuhan area.
“That’s why we have to shut it off and not give the virus any more opportunities,” he added.