The new coronavirus has been declared a global emergency by the World Health Organization, as the outbreak continues to spread outside China.
"The main reason for this declaration is not what is happening in China but what is happening in other countries," said WHO chief Tedros Adhanom Ghebreyesus.
The concern is that it could spread to countries with weaker health systems.
Meanwhile, the US has told its citizens not to travel to China.
The State Department issued a level four warning - having previously urged Americans to "reconsider" travel to China - and said any citizens in China "should consider departing using commercial means".
At least 213 people have died in China - mostly in Hubei province where the virus emerged - with almost 10,000 cases nationally.
The WHO said there had been 98 cases in 18 other countries, but no deaths.
Most international cases are in people who had been to the Chinese city of Wuhan in Hubei.
Coronavirus outbreak outside China
18 The number of countries with cases
14 Cases in Thailand and Japan
8 Australia and Malaysia
6 South Korea
5 France and USA
Source: WHO and local authorities
However, there have been eight cases of human-to-human infection - in Germany, Japan, Vietnam and the United States.
Speaking at a news conference in Geneva, Dr Tedros described the virus as an "unprecedented outbreak" that has been met with an "unprecedented response".
He praised the "extraordinary measures" Chinese authorities had taken, and said there was no reason to limit trade or travel to China.
"Let me be clear, this declaration is not a vote of no confidence in China," he said.
But various countries have taken steps to close borders or cancel flights, and companies like Google, Ikea, Starbucks and Tesla have closed their shops or stopped operations.
The US Commerce Secretary, Wilbur Ross, has said the outbreak could "accelerate the return of jobs to North America".