HONG KONG — A mysterious new respiratory virus that emerged in central China has killed at least three people and sickened more than 200 others, raising concerns about a deadly outbreak in the midst of the country's busiest travel period of the year.
On Monday, the authorities reported that new cases had been detected for the first time in Beijing, in Shanghai and the southern city of Shenzhen, all hundreds of miles from the city of Wuhan, where the virus first appeared. Cases have also been reported in Japan, South Korea and Thailand.
Here is what we know about the virus, where it has been found, how it is spreading and what precautions are being taken:
The virus emerged in the city of Wuhan.
The government of Wuhan, a city in central China, first confirmed on Dec. 31 that hospitals in the city were treating dozens of patients for pneumonia with an unknown cause.
Many of the cases were connected to the Huanan Seafood Market, which also sold live poultry and exotic animal meats. Considered a likely source of the virus, the market was closed and disinfected.
The health commission in Wuhan said on Sunday that the illness had also appeared in people who had not been exposed to the market, raising the possibility that the virus could be present elsewhere in the city.
Local officials have pledged to handle the outbreak with transparency. But the memory of how China initially covered up the extent of a deadly SARS outbreak that infected more than 8,000 people in 2002 and 2003 has not completely faded. Although flu experts have said the Chinese government is trying to be more transparent now, many in China remain skeptical.
Over the weekend, the number of reported cases more than tripled to about 200, mostly in Wuhan. One more person in the city died, bringing the total death toll to three, while nine others remained critically ill, the city's health commission said. Twenty-five people have recovered.
On Monday, five new cases were reported in Beijing, 14 in Guangdong Province, and one in Shanghai, the first cases that were reported in China outside Wuhan. The total number of cases climbed to at least 218, state media reported on Monday.
The World Health Organization said the spike in reported cases was the result of increased searching and testing of respiratory illnesses.
Cases have been reported outside China.
The authorities in Thailand detected the new coronavirus last week in two Chinese women who had flown from Wuhan to Bangkok on separate trips. The government said the women, aged 74 and 61, were in good condition.
In Japan, a Chinese man who returned from Wuhan on Jan. 6 was also confirmed to have the disease. He was discharged after five days in a hospital.
South Korea confirmed its first case of the coronavirus on Monday in a 35-year-old Chinese woman from Wuhan who arrived on Sunday at Incheon International Airport, which served Seoul.
The woman was found with a fever, muscle pain and other symptoms while going through customs and was immediately quarantined for tests, said Jung Eun-kyeong, director of the Korea Centers for Disease Control and Prevention.
The woman was traveling with five other people intending to spend the Lunar New Year holidays in South Korea and Japan, Ms. Jung said. South Korean officials were running tests anyone who was believed to have come in contact with the woman in the plane, she said.
Five people who traveled from Wuhan to Zhejiang, a coastal Chinese province south of Shanghai, are being treated for fever but have not been diagnosed with the new coronavirus, the health authorities there said.
State media also reported six suspected cases on Monday, including two in Sichuan Province, one in Shanghai, one in Yunnan Province, one in Shandong Province and one in the region of Guangxi.
In the southern city of Shenzhen, the authorities have started imposing temperature screening procedures at the airport and at train and bus stations. The city also said it would also crack down on the illegal trading of wild animals.
China's leader, Xi Jinping, said on Monday that the outbreak “must be taken seriously” and that every possible measure should be taken to contain it, according to the state broadcaster CCTV.
It doesn't appear to spread easily between humans.
Researchers in China identified the mysterious pneumonialike illness in early January as a new coronavirus. Experts said it does not appear to be readily spread by humans, but cautioned that more research was necessary.
Coronaviruses are named after the spikes that protrude from their membranes, like the sun's corona. Such viruses cause several illnesses of the respiratory tract, ranging from the common cold to severe diseases like SARS.
According to the World Health Organization, common signs of infection include fever, cough, and respiratory difficulties like shortness of breath. Serious cases can lead to pneumonia, kidney failure and even death.
Animals appear to be the most likely source.
The W.H.O. says animals appear to be the most likely primary source of the outbreak, though it is still not known which animals are responsible.
Past outbreaks of similar illnesses, like SARS, were believed to have emerged from markets where people and live animals were in regular contact.
The organization says it is possible that the virus could spread between people who are in close contact, but it was unclear whether the disease could spread easily between humans.
To prevent the spread of respiratory infections, the W.H.O. recommends that people wash their hands regularly, cover their mouths and noses when coughing and sneezing, and avoid direct contact with farm or wild animals.
The health authorities in Hong Kong have also advised residents traveling outside the city not to touch live animals, not to eat wild animals, and to avoid markets selling fresh meat and live poultry.
Choe Sang-Hun contributed reporting from Seoul, South Korea. Albee Zhang contributed research from Beijing.