Authorities are struggling to identify a mystery virus that's spreading in central China.
As of Sunday, 59 people had been infected, health officials in Wuhan said, and another 163 are under observation.
Seven of the infected are in a critical condition.
Initial fears that SARS, which killed 774 people in 2002-3, could be back have proven unfounded. Bird flu has also been ruled out, as has adenovirus and Middle East respiratory syndrome, with doctors simply calling it an "unknown viral pneumonia" at this stage.
There's no evidence yet it can be transmitted from person-to-person. Some of those infected worked at a seafood market in the sprawling city of 19 million, which the South China Morning Post reports has been shut down as a precaution.
The virus was first reported on December 31, 2019.
"The clinical signs and symptoms are mainly fever, with a few patients having difficulty in breathing, and chest radiographs showing invasive lesions of both lungs," the World Health Organisation (WHO) said on Sunday.
"The reported link to a wholesale fish and live animal market could indicate an exposure link to animals," the World Health Organization said on Sunday.
Airports in Hong Kong, Singapore and Taiwan have reportedly upped their screening of passengers travelling from China. Eight people who recently visited Wuhan have been hospitalised in Hong Kong after showing symptoms, the Hong Kong Hospital Authority said.
"Frontline healthcare staff of public hospitals have been reminded to pay special attention to clinical information, including the presentation of fever and acute respiratory illness, or pneumonia; and travel history to Wuhan within 14 days before onset of symptoms."
Despite the mysterious nature of the virus, the WHO isn't recommending against travel to Wuhan or China just yet.
"In case of symptoms suggestive of respiratory illness either during or after travel, travellers are encouraged to seek medical attention and share travel history with their healthcare provider."
In 2017, SARS was eventually traced back to bats living in a remote cave in Yunnan, which likely spread to humans in animal markets like the one in Wuhan.