By James Gallagher
A virus - previously unknown to science - is causing severe lung disease in China and has also been detected in other countries.
People are known to have died and the outbreak shows no sign of stopping soon.
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.
Can this outbreak be contained or is this something far more dangerous?
What is this virus?
Officials in China have confirmed the cases are caused by a coronavirus.
These 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.
How severe are the symptoms?
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 World Health Organization says data from 17,000 patients suggests 82% have mild disease, 15% are severe and 3% critical.
Notably, the infection rarely seems to cause a runny nose or sneezing.
The coronavirus family itself can cause symptoms ranging from a mild cold all the way through to severe lung problems, which can kill.
How deadly is it?
While the ratio of deaths to known cases appears low, the figures are unreliable.
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.
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.
And it is unclear how many unreported mild cases are out there - so the death rate could also be lower.
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.
Wash your hands - as soap or hand-gels can kill the virus
Cover your mouth and nose when coughing or sneezing - ideally into a tissue and wash hands afterwards to minimise spread
Avoid touching the eyes nose and mouth - if your hands touch a surface contaminated by the virus this could transfer the virus into your body
Don't get too close to people coughing, sneezing or have a fever - as they propel small droplets containing the virus into the air - ideally keep one metre away.
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.
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.
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?
There are thousands of new cases being reported each day.
However, outbreak analysts believe these are only the tip of the iceberg.
Their mathematical models suggest the true scale of the outbreak could be 10 times larger than the official figures.
Multiple groups have estimated the number of cases is doubling every five to seven days.
Is the virus mutating?
The virus seems to be quite stable, so far.
However, it is possible for viruses to mutate and that is something scientists will be watching closely.
Can the outbreak be stopped?
The World Health Organization say the outbreak can be contained, but this is not a universally shared view.
Experts, including the former head of the US Centers for Disease Control, argue the virus could become a pandemic - a global epidemic.
The only way it can be stopped is to prevent people who have become infected from spreading the virus to others.
limiting people's movement
encouraging hand-washing and other forms of infection control
treating patients in isolation with healthcare workers wearing protective gear
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?
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.
nd 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."