Lockdown measures have ramped up in China to contain a fast-spreading virus, with three cities under restrictions as more cases emerge around the world.
The World Health Organisation is poised to announce whether to declare the coronovirus (2019-nCoV) outbreak a global health emergency, describing the lockdown of millions of Chinese as "unprecedented".
It comes amid preliminary research suggesting humans may have contracted the respiratory illness from snakes which in turn may become infected hunting bats in the wild.
A new report in the Journal of Medical Virology says an analysis of the virus's genetics shows it is closely related to two types of SARS illness originating from bats and may have spread from bats to snakes – in particular the Chinese Krait and Chinese Cobra – and snakes to humans.
Snakes are apparently sold at the seafood market in Wuhan, a city of 11 million people in central China, which is the epic-centre of the outbreak.
The first people to fall ill were believed to have been staff or customers at the market where a range of animals products an illegal wildlife were sold, as well as seafood.
However Chinese government medical adviser Zhong Nanshan has also identified badgers and rats as possible sources.
Health officials fear the transmission rate of the SARS-like infection will accelerate as hundreds of millions of Chinese travel at home and abroad during week-long holidays for the Lunar New Year, which begins on Saturday.
Airports around the world have begun screening processes as cases of the virus are confirmed in Thailand (four), Vietnam (two), and one each in Singapore, Japan, South Korea, Taiwan and the United States.
Singapore confirmed its first case was a Wuhan man who arrived in the country on January 20 and is currently in isolation in hospital.
One of his travelling companions has also been hospitalised as a suspect case.
Vietnam's health ministry said on Thursday two Chinese citizens in the southeast Asian country had tested positive for coronavirus, but were in a "good condition".
Most transport in Wuhan was suspended on Thursday and people were told not to leave.
Hours later, neighbouring Huanggang, a city of about 7 million people, announced a similar lockdown while the city of Ezhou has placed restrictions on public transport.
As Wuhan slipped into isolation, residents thronged into hospitals for checks and scrambled for supplies, clearing out supermarket shelves and queuing for petrol.
The World Health Organisation's representative in Beijing, Gauden Galea, said the lockdown of 11 million people was unprecedented in public health history.
Other cities were also taking steps to restrict movement and contact.
China's Education Ministry said schools should not hold large events or exams.
The capital cancelled major public events, including two Lunar New Year temple fairs, the state-run Beijing News said.
Hong Kong, which has two confirmed cases, is turning two holiday camps into quarantine stations as a precaution. Taiwan has banned anyone from Wuhan from going to the island.
There is no vaccine for the virus, which can spread through respiratory transmission. Symptoms include fever, difficulty in breathing and cough, similar to many other respiratory illnesses.
The WHO will decide on Thursday whether to declare the outbreak a global health emergency, which would step up the international response.
If it does so, it will be the sixth international public health emergency to be declared in the last decade.
Some experts, including Australia's chief medical officer, believe the virus is not as dangerous as previous coronaviruses such as Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome.
"The early evidence at this stage would suggest it's not as severe," Australia's Chief Medical Officer Brendan Murphy said.
In a report on Wednesday, Imperial College London said it estimated a total of 4000 cases of the coronavirus in Wuhan alone as of January 18, an infection rate based on the number of cases reported in China and elsewhere.
In contrast with its secrecy over the 2002-03 Severe Acute Respiratory Syndrome (SARS) that killed nearly 800 people, China's Communist Party government has provided regular updates to avoid panic ahead of the holidays.