Someone who is infected with the coronavirus can spread it with just a simple cough or a sneeze, scientists say.
At least 1,370 people with the virus are now confirmed to have died and more than 60,380 have been infected in at least 28 countries and regions. But experts predict the true number of people with the disease could be 100,000, or even as high as 350,000 in Wuhan alone, as they warn it may kill as many as two in 100 cases. Here's what we know so far:
What is the coronavirus?
A coronavirus is a type of virus which can cause illness in animals and people. Viruses break into cells inside their host and use them to reproduce itself and disrupt the body's normal functions. Coronaviruses are named after the Latin word 'corona', which means crown, because they are encased by a spiked shell which resembles a royal crown.
The coronavirus from Wuhan is one which has never been seen before this outbreak. It has been named SARS-CoV-2 by the International Committee on Taxonomy of Viruses. The name stands for Severe Acute Respiratory Syndrome coronavirus 2.
Experts say the bug, which has killed around one in 50 patients since the outbreak began in December, is a 'sister' of the SARS illness which hit China in 2002, so has been named after it.
The disease that the virus causes has been named COVID-19, which stands for coronavirus disease 2019.
Dr Helena Maier, from the Pirbright Institute, said: 'Coronaviruses are a family of viruses that infect a wide range of different species including humans, cattle, pigs, chickens, dogs, cats and wild animals.
'Until this new coronavirus was identified, there were only six different coronaviruses known to infect humans. Four of these cause a mild common cold-type illness, but since 2002 there has been the emergence of two new coronaviruses that can infect humans and result in more severe disease (Severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS) coronaviruses).
'Coronaviruses are known to be able to occasionally jump from one species to another and that is what happened in the case of SARS, MERS and the new coronavirus. The animal origin of the new coronavirus is not yet known.'
The first human cases were publicly reported from the Chinese city of Wuhan, where approximately 11million people live, after medics first started publicly reporting infections on December 31.
By January 8, 59 suspected cases had been reported and seven people were in critical condition. Tests were developed for the new virus and recorded cases started to surge.
The first person died that week and, by January 16, two were dead and 41 cases were confirmed. The next day, scientists predicted that 1,700 people had become infected, possibly up to 7,000.
Just a week after that, there had been more than 800 confirmed cases and those same scientists estimated that some 4,000 – possibly 9,700 – were infected in Wuhan alone. By that point, 26 people had died.
By January 27, more than 2,800 people were confirmed to have been infected, 81 had died, and estimates of the total number of cases ranged from 100,000 to 350,000 in Wuhan alone.
By January 29, the number of deaths had risen to 132 and cases were in excess of 6,000.
By February 5, there were more than 24,000 cases and 492 deaths.
By February 11, this had risen to more than 43,000 cases and 1,000 deaths.
A change in the way cases are confirmed on February 13 – doctors decided to start using lung scans as a formal diagnosis, as well as laboratory tests – caused a spike in the number of cases, to more than 60,000 and to 1,369 deaths.
Where does the virus come from?
According to scientists, the virus has almost certainly come from bats. Coronaviruses in general tend to originate in animals – the similar SARS and MERS viruses are believed to have originated in civet cats and camels, respectively.
The first cases of COVID-19 came from people visiting or working in a live animal market in the city, which has since been closed down for investigation.
Although the market is officially a seafood market, other dead and living animals were being sold there, including wolf cubs, salamanders, snakes, peacocks, porcupines and camel meat.
A study by the Wuhan Institute of Virology, published in February 2020 in the scientific journal Nature, found that the genetic make-up virus samples found in patients in China is 96 per cent similar to a coronavirus they found in bats.
However, there were not many bats at the market so scientists say it was likely there was an animal which acted as a middle-man, contracting it from a bat before then transmitting it to a human. It has not yet been confirmed what type of animal this was.
Dr Michael Skinner, a virologist at Imperial College London, was not involved with the research but said: 'The discovery definitely places the origin of nCoV in bats in China.
'We still do not know whether another species served as an intermediate host to amplify the virus, and possibly even to bring it to the market, nor what species that host might have been.'
So far the fatalities are quite low. Why are health experts so worried about it?
Experts say the international community is concerned about the virus because so little is known about it and it appears to be spreading quickly.
It is similar to SARS, which infected 8,000 people and killed nearly 800 in an outbreak in Asia in 2003, in that it is a type of coronavirus which infects humans' lungs.
Another reason for concern is that nobody has any immunity to the virus because they've never encountered it before. This means it may be able to cause more damage than viruses we come across often, like the flu or common cold.
Speaking at a briefing in January, Oxford University professor, Dr Peter Horby, said: 'Novel viruses can spread much faster through the population than viruses which circulate all the time because we have no immunity to them.
'Most seasonal flu viruses have a case fatality rate of less than one in 1,000 people. Here we're talking about a virus where we don't understand fully the severity