A new SARS-like virus has killed six people in China and infected nearly 300 others.
Fears have been mounting that the virus will spread during the massive annual Lunar New Year migration. A host of Asian countries and the United States have introduced new screening checks for passengers from Wuhan, the Chinese city identified as the epicentre.
Here's what we know about the virus:
IT'S ENTIRELY NEW
The virus appears to be a never-before-seen strain of coronavirus -- a large family of viruses that can cause diseases ranging from the common cold to Severe Acute Respiratory Syndrome (SARS), which killed 349 people in mainland China and another 299 in Hong Kong between 2002 and 2003.
Arnaud Fontanet, head of the department of epidemiology at the Institut Pasteur in Paris, told AFP the current virus strain was 80 percent genetically identical to SARS.
China has already shared the genome sequencing of this novel coronavirus with the international scientific community.
For now, it is being dubbed "2019-nCoV".
IT'S BEING PASSED BETWEEN HUMANS
The World Health Organization said Monday it believed an animal source was the "primary source" of the outbreak, and Wuhan authorities identified a seafood market as the centre of the epidemic.
But China has since confirmed that there was evidence the virus is now passing from person to person, without any contact with the market.
Doctor Nathalie MacDermott of King's College London said it seems likely that the virus is spread through droplets in the air from sneezing or coughing.
Doctors at the University of Hong Kong published an initial paper Tuesday modelling the spread of the virus which estimated that there have been some 1,343 cases in Wuhan -- similar to a projection of 1,700 last week by Imperial College, London.
Both are much higher than official figures.
IT IS MILDER THAN SARS
Compared with SARS, the symptoms appear to be less aggressive, and experts say the death toll is still relatively low.
According to authorities in Wuhan, 25 of the more than 200 people infected in the city have already been discharged.
"It's difficult to compare this disease with SARS," said Zhong Nanshan, a renowned scientist at China's National Health Commission at a press conference this week. "It's mild. The condition of the lung is not like SARS."
However, the milder nature of the virus can also cause alarm.
The outbreak comes as China prepares for the Lunar New Year Holiday, with hundreds of millions travelling across the country to see family.
Professor Antoine Flahault, director of the Institute of Global Health at the University of Geneva, told AFP that the fact that the virus seems milder in the majority of people is "paradoxically more worrying" as it allows people to travel further before their symptoms are detected.
"Wuhan is a major hub and with travel being a huge part of the fast approaching Chinese New Year, the concern level must remain high," said Dr Jeremy Farrar, Director of the Wellcome Trust.
INTERNATIONAL PUBLIC HEALTH EMERGENCY?
The WHO will hold a meeting on Wednesday to determine whether the outbreak constitutes a "public health emergency of international concern" and if so, what should be done to manage it.
The agency has only used the rare label a handful of times, including during the H1N1 -- or swine flu -- pandemic of 2009 and the Ebola epidemic that devastated parts of West Africa from 2014 to 2016.
The Chinese government announced Tuesday it was classifying the outbreak in the same category as the SARS outbreak, meaning compulsory isolation for those diagnosed with the disease and the potential to implement quarantine measures on travel.
But if the WHO decides to take this step, it would put the Wuhan virus in the same category as a handful of very serious epidemics.
US CONFIRMS FIRST CASE OF CHINA VIRUS ON AMERICAN SOIL
US health authorities on Tuesday announced the first case of a person on American soil sickened by a new virus that emerged in the central Chinese city of Wuhan, and intensified airport health screenings.
The man, a US resident in his 30s who lives near Seattle, is in good condition, according to federal and state officials.
The man is being "hospitalized out of an abundance of precaution, and for short term monitoring, not because there was severe illness," said Chris Spitters, a Washington state health official.
The news came as Asian countries ramped up measures to block the spread of the new virus as the death toll in China rose to six and the number of cases surpassed 300.
China is about to embark on the Lunar New Year holiday -- a huge travel time for hundreds of millions of people.
The man hospitalized in Washington state had traveled to the US from Wuhan, but did not visit the seafood market thought to be at the heart of the outbreak.
He entered the US on January 15 -- two days before health screenings for those traveling from Wuhan began at airports in Los Angeles, New York and San Francisco -- and approached health authorities himself after reading news reports about the virus.
Officials from the Centers for Disease Control and Prevention (CDC) will now be deployed to two additional international airports, in Chicago and Atlanta.
All travelers on flights from Wuhan, either direct or connecting, will now be re-routed to the five airports with screening systems in place.
Nations across the Asia-Pacific region stepped up checks of passengers at airports to detect the SARS-like coronavirus.
Fears of a bigger outbreak rose after a prominent expert from China's National Health Commission confirmed late Monday that the virus can be passed between people.
MACAU CONFIRMS VIRUS CASE AND ORDERS CASINO STAFF TO MASK UP
Macau on Wednesday reported its first confirmed case of the new SARS-like coronavirus as authorities announced all staff in the city's bustling casinos had been ordered to wear face masks.
The former Portuguese colony is a huge draw for mainland tourists as the only place in China that allows gambling.
With the Lunar New Year approaching this weekend, a huge influx of mainland tourists is expected in the city.
Asian countries have ramped up measures to block the spread of the new virus, which emerged in the Chinese city of Wuhan and has so far killed at least nine people.
On Wednesday, Macau announced its first confirmed case -- a 52-year-old businesswoman from Wuhan who arrived in the city by high-speed rail on Sunday, via the neighbouring city of Zhuhai.
"A series of tests found that she was positive for the coronavirus and had symptoms of pneumonia," Lei Chin-lon, the head of Macau's health bureau, told reporters.
The woman had been staying at the New Orient Landmark Hotel with two friends who were being monitored since her admission to hospital on Tuesday.
Ao Ieong Iu, Macau's Secretary for Social Affairs and Culture, said staff in all casinos would be required to wear masks while anyone arriving at entry ports along the city's border with the mainland would need to fill out health declaration forms.
"We have not banned tourism groups from Wuhan but we are not encouraging them," Ao Ieong said.
"We will stay in close contact with tourism agencies and require them to notify us of all groups going to and coming from Wuhan," she added.
NORTH KOREA TO BAN TOURISTS OVER CHINA VIRUS
North Korea will ban foreign tourists to protect itself against a new SARS-like virus that has claimed at least nine lives in China and sickened hundreds, a major tour operator said.
Several nations including the US have stepped up checks on airport passengers to detect the coronavirus, which first emerged in the central Chinese city of Wuhan, but a ban on tourists would be a first.
The virus has caused alarm in China and abroad because of its genetic similarities to Severe Acute Respiratory Syndrome (SARS), which killed nearly 650 people across mainland China and Hong Kong in 2002-2003.
The vast majority of foreign tourists to North Korea are Chinese, and their numbers have swelled significantly in the last year as relations between Beijing and Pyongyang have warmed, generating vital foreign currency for the isolated country.
But as of Wednesday, North Korea will "temporarily close its borders to all foreign tourists as a precaution to the coronavirus", Chinese-based specialist operator Young Pioneer Tours said in a statement.
Exact details had yet to be confirmed, it added.
Koryo Tours, the market leader for Western visitors, said it had been "informed that action was being considered" and was expecting more information Wednesday.
Pyongyang has taken similar action in the past -- it closed its borders to foreign tourists for more than four months from October 2014 in a bid to keep out the Ebola virus, even though no cases had been reported in Asia.
It enforced a 21-day quarantine period on anyone entering the country, including foreign diplomats and businesspeople.
Pyongyang's official Rodong Sinmun newspaper did not mention any North Korean action over the virus Wednesday but reported on the outbreak in China, saying it had "spread rapidly" and that authorities in its giant neighbour were taking "corresponding measures".
The North has poor medical infrastructure and a chronic shortage of medicines.
When South Korea suffered from the world's largest outbreak of Middle East Respiratory Syndrome, or MERS, in summer 2015, the North reportedly prohibited its diplomats and workers overseas from returning to their homeland for months to prevent the spread of the virus.
Pyongyang also suspended foreign tours for three months due to fears over SARS in 2003.