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.
The death toll now stands at 170 people in China.
The WHO said there had been 98 cases in 18 countries outside of the country, but no deaths.
Most cases have emerged in people who have travelled from the Chinese city of Wuhan, where the outbreak began.
However, there have been eight cases of human-to-human infection – in Germany, Japan, Vietnam and the United States.
Dr Tedros, speaking at the press conference in Geneva, described the virus as an "unprecedented outbreak" that has been met with an "unprecedented response".
He praised the "extraordinary measures" Chinese authorities had taken to prevent it from spreading.
"Let me be clear, this declaration is not a vote of no confidence in China," he added.
BBC Health correspondent James Gallagher said the WHO will now be able to support lower and middle income countries, helping them strengthen their disease surveillance and prepare them for possible cases.
Researchers are racing to develop a vaccine to protect people from the virus. One lab in California has plans for a potential vaccine to enter human trials by June or July.
How unusual is this declaration?
The WHO declares a Public Health Emergency of International Concern when there is "an extraordinary event which is determined … to constitute a public health risk to other states through the international spread of disease".
It has previously declared five global public health emergencies:
- Swine flu, 2009 –The H1N1 virus spread across the world in 2009, killing more than 200,000 people, and a public health emergency was called to ensure the world was carefully monitoring its spread and able to respond, including with vaccines.
- Polio, 2014 – Although closer than ever to eradication in 2012, polio numbers rose in 2013. An emergency was declared due to fears the global fight against its eradication could face a major setback.
- Zika, 2016 – The WHO declared Zika a public health emergency in 2016 after the disease spread rapidly through the Americas. Although for many Zika symptoms are mild, it can be dangerous for pregnant women and the emergency was called to spur urgent research.
- Ebola, 2014 and 2019 – The deadly disease has twice been declared a public health emergency. The first one lasted from August 2014 to March 2016 as almost 30,000 people were infected and more than 11,000 died in West Africa. The WHO cited "the virulence of the virus, the intensive community and health facility transmission patterns, and the weak health systems" in affected countries. A second emergency was declared last year as an outbreak spread in DR Congo.
How is China handling the outbreak?
A confirmed case in Tibet means the virus has now reached every region in mainland China.
Although questions have been raised about transparency, the WHO has praised China's handling of the outbreak. President Xi Jinping has vowed to defeat what he called a "devil" virus.
The central province of Hubei, where nearly all deaths have occurred, is in a state of lockdown. The province of 60 million people is home to Wuhan, the heart of the outbreak.
The city has effectively been sealed off and China has put numerous transport restrictions in place to curb the spread of the virus.
People who have been in Hubei are also being told by their employers to work from home until it is considered safe for them to return.
The virus is affecting China's economy, the world's second-largest, with a growing number of countries advising their citizens to avoid all non-essential travel to the country.
Several international airlines have stopped or scaled back their routes to China and companies like Google, Ikea, Starbucks and Tesla have closed their shops or stopped operations.
How is the world responding?
Voluntary evacuations of hundreds of foreign nationals from Wuhan are under way to help people who want to leave the closed-off city and return to their countries.
The UK, Australia, South Korea, Singapore anRead More – Source