A new coronavirus, a mysterious SARS-like disease, has spread around China and three other Asian countries since first emerging in the central Chinese city of Wuhan.
Patients are quarantined and work begins on identifying the origin of the pneumonia.
The US Centers for Disease Control and Prevention (CDC) identifies a seafood market suspected to be at the centre of the outbreak. It is closed on 1 January 2020.
To date 59 people have been infected of whom seven are in a serious condition, according to an official toll.
The Chinese health authorities say a first person has died of the virus on January 11.
They revise downwards the number of sick people to 41.
Spreads beyond China
On January 13, the virus spreads beyond China's borders for the first time with a case emerging in Thailand, according to the WHO. The victim is a Chinese woman diagnosed with mild pneumonia who was returning from a trip to Wuhan.
On January 15 China's health commission says no human-to-human transmission of the virus behind the Wuhan outbreak has been confirmed so far, but the possibility "cannot be excluded".
The next day a first case of the virus is confirmed in Japan in someone who had stayed in Wuhan in early January.
On January 17, a second person, a 69-year-old man, dies in Wuhan, according to the authorities.
The same day, the CDC announces that it will begin screening passengers arriving from Wuhan at three airports: San Francisco, New York's JFK and Los Angeles.
Human to human transmission confirmed
On January 20, a third death and more than 100 new cases are announced in China, sparking concerns ahead of the annual Lunar New Year holiday which begins January 25 and sees hundreds of millions of Chinese people travel nationwide.
The virus is present in Beijing in the north, Shanghai in the east and Shenzhen in the south. More than 200 cases have been recorded. The virus is also detected in South Korea in a Chinese person who has arrived by plane from Wuhan.
China's President Xi Jinping says that the virus must be "resolutely contained", in his first public comments on the outbreak.
Human-to-human transmission is "affirmative", a top Chinese expert on infectious diseases Zhong Nanshan tells state broadcaster CCTV.