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 Jeremy Farrar, Director of the Wellcome Trust.
It could lead to an 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.