A virus – previously unknown to science – is causing severe lung disease in the Chinese city of Wuhan and has been detected in two other countries.
Four people are known to have died from the virus, which appeared in the city in December.
There are more than two hundred confirmed cases of the virus, but UK experts estimate the figure is closer to 1,700.
A new virus arriving on the scene, leaving patients with pneumonia, is always a worry and health officials around the world are on high alert.
But is this a brief here-today-gone-tomorrow outbreak or the first sign of something far more dangerous?
What is this virus?
Viral samples have been taken from patients and analysed in the laboratory.
And officials in China and the World Health Organization (WHO) have concluded the infection is a coronavirus.
Coronaviruses are a broad family of viruses, but only six (the new one would make it seven) are known to infect people.
Severe acute respiratory syndrome (Sars), which is caused by a coronavirus, killed 774 of the 8,098 people infected in an outbreak that started in China in 2002.
“There is a strong memory of Sars, that’s where a lot of fear comes from, but we’re a lot more prepared to deal with those types of diseases,” says Dr Josie Golding, from the Wellcome Trust.
Is it serious?
Coronaviruses can cause symptoms ranging from a mild cold all the way through to death.
This new virus appears to be somewhere in the middle.
“When we see a new coronavirus, we want to know how severe are the symptoms – this is more than cold-like symptoms and that is a concern but it is not as severe as Sars,” says Prof Mark Woolhouse, from the University of Edinburgh.
The World Health Organization (WHO) is considering declaring an international public health emergency over the virus – as it did with swine flu and Ebola.
Where has it come from?
New viruses are detected all the time.
They jump from one species, where they went unnoticed, into humans.
“If we think about outbreaks in the past, if it is a new coronavirus, it will have come from an animal reservoir,” says Prof Jonathan Ball, a virologist at the University of Nottingham.
Sars jumped from the civet cat into humans.
And Middle East respiratory syndrome (Mers), which has killed 858 out of the 2,494 recorded cases since it emerged in 2012, regularly makes the jump from the dromedary camel.
Once the animal reservoir where the virus normally camps out is detected, the problem becomes much easier to deal with.
The cases have been linked to the South China Seafood Wholesale Market, in Wuhan.
But while some sea-going mammals can carry coronaviruses (such as the Beluga whale), the market also has live wild animals, including chickens, bats, rabbits, snakes, which are more likely to be the source.
Prof Woolhouse says it is because of the size and density of the population and close contact with animals harbouring viruses.
“No-one is surprised the next outbreak is in China or that part of the world,” he says.
How easily does it spread?
Chinese officials say there have been cases of the virus spreading from one person to another, with some healthcare workers becoming infected after caring for patients with the same illness.
This is a concern with new viruses that infect the lungs, as coughs and sneezes are a highly effective way for a virus to spread.
It is too soon know how many people might become ill.
How fast is it spreading?
The outbreak was thought to be limited, but new cases have been reported since it started in December.
While the outbreak is centred on Wuhan, there have been two cases in Thailand, one in Japan and another in South Korea. Those people travelled from Wuhan recently.
Experts say there could be more cases going undetected.
A report by the MRC Centre for Global Infectious Disease Analysis at Imperial College London suggested there could be more than 1,700 infections.
“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,” said the report.
There are concerns that the virus could be spread by the hundreds of millions of people travelling for Chinese New Year later this month.
Singapore and Hong Kong have been screening air passengers from Wuhan and US authorities announced similar measures starting on Friday at three major airports in San Francisco, Los Angeles and New York.
How have Chinese authorities responded?
Infected people have been treated in isolation to minimise the risk of the bug spreading.
Extra checks such as temperature scans have been put in place to screen travellers.
And the seafood market was closed for cleaning and disinfection.
The US and most Asian countries have stepped up screenings of travellers from Wuhan and the WHO has warned hospitals worldwide that a wider outbreak is possible.
How worried are the experts?
Dr Golding says: “At the moment, until we have more information, it’s really hard to know how worried we should be.
“Until we have confirmation of the source, that’s always going to make us uneasy.”
Prof Ball says: “We should be worried about any virus that explores humans for the first time, because it’s overcome the first major barrier.
“Once inside a [human] cell and replicating, it can start to generate mutations that could allow it to spread more efficiently and become more dangerous.
“You don’t want to give the virus the opportunity.”