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".
China has said it will send charter plans to bring back Hubei province residents who are overseas "as soon as possible". A foreign ministry spokesman said this was because of the "practical difficulties" Chinese citizens have faced abroad. Hubei is where the virus emerged. At least 213 people in China have died from the virus, mostly in Hubei, with almost 10,000 cases nationally.
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. 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.
Preparing other countries What happens if this virus finds its way into a country that cannot cope? Many low and middleincome countries simply lack the tools to spot or contain it. The fear is it could spread uncontrollably and that it may go unnoticed for some time.
Remember this is a disease which emerged only last month - and yet there are already almost 10,000 confirmed cases in China. But declaring a global emergency allows the WHO to support lower and middle-income countries to strengthen their disease surveillance - and prepare them for cases. .