What we know so far

It's entirely new: The pathogen appears to be a never-before-seen strain of coronavirus -- a 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. The new virus has been named "2019-nCoV". Most patients experience flu-like symptoms, including fever, a cough, shortness of breath, a sore throat or runny nose.

Where did it come from?: It may have originated in bats, according to genetic analysis, but researchers say there could have been an "intermediate host" in the transmission to humans that one study suggested could have been snakes. Gao Fu, director of the Chinese Center for Disease Control and Prevention, said on Wednesday the virus likely came from "wild animals at a seafood market" in Wuhan. The market offered a range of exotic wildlife for sale, including live foxes, crocodiles, wolf pups, giant salamanders, snakes, rats, peacocks, porcupines and camel meat. SARS was linked to Chinese consumption of civet meat.

Human to human transmission: China has confirmed the virus is passing from person to person without any contact with the now-closed market. While most patients are in Wuhan, cases have been detected across China and a few abroad. Nathalie MacDermott of King's College London said it seems likely the virus is spreading through droplets in the air from sneezing or coughing. Scientists at Imperial College London published an estimate on Wednesday that 4,000 people had been infected in Wuhan -- around 10 times the official figure.

It is milder than SARS: The symptoms appear to be less aggressive than those of the virus that spread in 2002 and 2003. However, Antoine Flahault, director of the Institute of Global Health at the University of Geneva, told AFP the fact that the virus seems milder in the majority of people is "paradoxically more worrying" as it allows many to travel further before their symptoms are detected. The 17 patients who died were between 48 and 89 years old.

International public health emergency?: The World Health Organization on Wednesday postponed its decision on whether to declare a global public health emergency, extending talks by a day. It 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. Cases have so far been confirmed in Thailand, Japan, Taiwan, South Korea, Macau and the United States. The Chinese government said Tuesday it was classifying the outbreak in the same category as SARS, meaning compulsory isolation for those diagnosed and the potential to implement quarantine measures on travel.

Global precautions: Authorities halted flights and trains out of Wuhan from yesterday and told people in the city they should not leave without a special reason. In Thailand, officials have introduced mandatory thermal scans of passengers arriving at airports in Bangkok, Chiang Mai, Phuket and Krabi from high-risk areas in China. In Hong Kong, authorities have said they are on high alert, carrying out scans at the city's airport and at other international land and sea crossing points.  Taiwan has issued travel advisories. South Korea urged its citizens yesterday not to travel to Wuhan ". The US has also ordered the screening of passengers arriving on direct or connecting flights from Wuhan. In Europe, Britain and Italy introduced enhanced monitoring of flights from Wuhan, while Romania and Russia are also strengthening checks.