The coronavirus, which surfaced in a Chinese seafood and poultry market late last year, has spread to at least 177 countries, killing more than 610,200 and sickening more than 14.7 million in a matter of months. The World Health Organization has declared the situation a pandemic.

Here’s a timeline of the outbreak so far.

Dec. 31

Chinese authorities treated dozens of cases of pneumonia of unknown cause.

On Dec. 31, the government in Wuhan, China, confirmed that health authorities were treating dozens of cases. Days later, researchers in China identified a new virus that had infected dozens of people in Asia. At the time, there was no evidence that the virus was readily spread by humans. Health officials in China said they were monitoring it to prevent the outbreak from developing into something more severe.

Jan. 11

China reported its first death.

On Jan. 11, Chinese state media reported the first known death from an illness caused by the virus, which had infected dozens of people. The 61-year-old man who died was a regular customer at the market in Wuhan. The report of his death came just before one of China’s biggest holidays, when hundreds of millions of people travel across the country.

Jan. 20

Other countries, including the United States, confirmed cases.

The first confirmed cases outside mainland China occurred in Japan, South Korea and Thailand, according to the W.H.O.’s first situation report. The first confirmed case in the United States came the next day in Washington State, where a man in his 30s developed symptoms after returning from a trip to Wuhan.