A virus - previously unknown to science - is causing severe lung disease in China and has also been detected in other countries.
A virus causing severe lung disease that started in China has spread to other countries, including the UK.
People are known to have died and the outbreak shows no sign of stopping soon.
The coronavirus had infected 59,864 people in China as of 13 February, with 1,368 of them dying.
A new virus arriving on the scene, leaving patients with pneumonia, is always a worry and health officials around the World Health Organization has declared a global emergency.
What are the symptoms?
Can this outbreak be contained or is this something far more dangerous?
It seems to start with a fever, followed by a dry cough.
What is this virus?
After a week, it leads to shortness of breath and some patients require hospital treatment. Notably, the infection rarely seems to cause a runny nose or sneezing.
Officials in China have confirmed the cases are caused by a coronavirus.
The incubation period - between infection and showing any symptoms - lasts up to 14 days, according to the World Health Organization (WHO).
These are a broad family of viruses, but only six (the new one would make it seven) are known to infect people.
But some researchers say it may be as long as 24 days.
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.
And Chinese scientists say some people may be infectious even before their symptoms appear.
"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.
How deadly is the coronavirus?
How severe are the symptoms?
Based on data from 17,000 patients with this coronavirus, the WHO says:
It seems to start with a fever, followed by a dry cough and then, after a week, leads to shortness of breath and some patients needing hospital treatment.
The proportion dying from the disease, which has been named Covid-19, appears low (between 1% and 2%) - but the figures are unreliable.
The World Health Organization says data from 17,000 patients suggests 82% have mild disease, 15% are severe and 3% critical.
Thousands are still being treated but may go on to die - so the death rate could be higher.
Notably, the infection rarely seems to cause a runny nose or sneezing.
But it is also unclear how many mild cases remain unreported - so the death rate could also be lower.
The disease is to be called Covid-19, the WHO announced.
To put this it into context, about one billion people catch influenza every year, with between 290,000 and 650,000 deaths. The severity of flu changes every year.
The coronavirus family itself can cause symptoms ranging from a mild cold all the way through to severe lung problems, which can kill.
Can coronavirus be treated or cured?
How deadly is it?
Right now, treatment relies on the basics - keeping the patient's body going, including breathing support, until their immune system can fight off the virus.
While the ratio of deaths to known cases appears low, the figures are unreliable.
However, the work to develop a vaccine is under way and it is hoped there will be human trials before the end of the year.
It is far too simplistic to divide the number of deaths by the number of cases to calculate the death rate to get a figure of around 2% at this stage of the outbreak.
Hospitals are also testing anti-viral drugs to see if they have an impact.
Thousands of patients are still being treated and we do not know if any of those cases will die - so the death rate could be higher.
How can I protect myself?
And it is unclear how many unreported mild cases are out there - so the death rate could also be lower.
The WHO says:
How can I reduce my risk of being infected?
The World Health Organization has the following advice, which apply to many other respiratory diseases too.
Where has it come from?
This virus is not really "new" - it is just new to humans, having jumped from one species to another.
"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.
Many of the early coronavirus cases were linked to the South China Seafood Wholesale Market, in Wuhan.
Sars started off in bats and then infected the civet cat, which in turn passed it on to 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.
Which animal?
While some sea-going mammals can carry coronaviruses (such as the Beluga whale), the South China Seafood Wholesale Market also had live wild animals, including chickens, rabbits, snakes, which are more likely to be the source.
The new virus is closely related to one found in Chinese horseshoe bats.
However, South China Agriculture University has suggested the virus could have moved from bats to pangolins and finally to humans.
Why China?
Two reasons - people coming into close contact with animals harbouring viruses and dense urban populations that allow it to spread.
"No-one is surprised the next outbreak is in China or that part of the world," says Prof Mark Woolhouse, from the University of Edinburgh.
How easily does it spread between people?
It largely spreads when infected people cough droplets containing the virus into the air that infect those nearby.
Initially, the Chinese authorities said the virus was not spreading between people.
But now we know each infected person is passing the virus on to between two and three other people.
This figure is the basic reproduction number of the virus - anything higher than one means the virus is self-sustaining.
In other words, this is not an outbreak that will burn out on its own and disappear.
When are people infectious?
Mostly when people are coughing, however, Chinese scientists have suggested some people may be infectious even before their symptoms appear.
The time between infection and symptoms - known as the incubation period - lasts up to 14 days.
Sars and Ebola are contagious only when symptoms appear. Such outbreaks are relatively easy to stop: identify and isolate people who are sick and monitor anyone they came into contact with.
But "symptomless spreaders" are much harder to stop.
How fast is it spreading?
How fast is it spreading?
There are thousands of new cases being reported each day.
Thousands of new cases are being reported each day.
However, outbreak analysts believe these are only the tip of the iceberg.
However, analysts believe the true scale could be 10 times larger than official figures.
Their mathematical models suggest the true scale of the outbreak could be 10 times larger than the official figures.
The number of cases is thought to be doubling every five to seven days.
Multiple groups have estimated the number of cases is doubling every five to seven days.
The WHO says the outbreak, which it has declared a global emergency, can be contained.
Why a global emergency?
But some experts, including a former head of the US Centers for Disease Control, say it could become a pandemic - a global epidemic.
The World Health Organization (WHO) says the virus is a public health emergency of international concern - as it did with swine flu and Ebola.
With colds and flu tending to spread fastest in the winter, there is hope the turning of the seasons may help stem the outbreak.
It said it has done so in order to provide extra support to lower and middle-income countries with weaker health systems that might not be able to spot or isolate cases of coronavirus.
School holidays may also help to slow its spread.
Is the virus mutating?
However, a different strain of coronavirus - Middle East respiratory syndrome - emerged in the summer, in Saudi Arabia, so there's no guarantee warmer weather will halt the outbreak.
The virus seems to be quite stable, so far.
How did it start?
However, it is possible for viruses to mutate and that is something scientists will be watching closely.
This virus is not really "new" - it is just new to humans, having jumped from one species to another.
Can the outbreak be stopped?
Many of the early cases were linked to the South China Seafood Wholesale Market, in Wuhan.
The World Health Organization say the outbreak can be contained, but this is not a universally shared view.
In China, a lot of people come into close contact with animals harbouring viruses - and the country's dense urban population means the disease can be easily spread.
Experts, including the former head of the US Centers for Disease Control, argue the virus could become a pandemic - a global epidemic.
Severe acute respiratory syndrome (Sars), which is also caused by a coronavirus, started off in bats and then infected the civet cat, which in turn passed it on to humans.
The only way it can be stopped is to prevent people who have become infected from spreading the virus to others.
The Sars outbreak, which started in China in 2002, killed 774 of the 8,098 people infected.
That means:
The current virus - one of seven types of coronavirus - does not seem to be mutating so far. But while it appears stable, this is something scientists will be watching closely.
A massive feat of detective work will also be needed to identify people who have been in close contact with patients to see if they have the virus.
Are there any vaccines or treatments?
No.
However, the work to develop them is already under way and it is hoped there will be human trials before the end of the year.
And hospitals are testing anti-viral drugs to see if they have an impact.
A combination of two drugs - lopinavir and ritonavir - was successful in the Sars epidemic and is being tested in China during this outbreak.
But at the moment, treatment relies on the basics - keeping the patient's body going, including breathing support, until their immune system can fight the virus off.
Can summer save us?
Colds and flu tend to do their main business in the winter months, so there is hope that the turning of the seasons may help stem the outbreak.
"Schools are closed for a long time in the summer and that helps limit the spread of many of these respiratory infections," said Prof John Edmunds from the London School of Hygiene and Tropical Medicine.
However, Mers-coronavirus emerged in the summer in Saudi Arabia.
"These viruses can certainly spread during the high temperature seasons, but the extent of the spread is what's important," said Prof David Heymann.
How have Chinese authorities responded so far?
China has done something unprecedented anywhere in the world - effectively putting entire cities into quarantine.
The central province of Hubei, which includes Wuhan, is in lockdown with around 60 million people affected.
Beijing has banned group dining for events such as birthdays and weddings while cities such as Hangzhou and Nanchang are limiting how many family members can leave home each day.
Hubei province has even switched off lifts in high-rise buildings to discourage residents from going outside.
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."
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