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.

Also on Monday, the authorities reported that new cases had been detected for the first time in Beijing, Shanghai and the southern province of Guangdong, all hundreds of miles from Wuhan. Cases have also been reported in Japan, South Korea and Thailand. In many of these cases, people infected with the virus had traveled to Wuhan.

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 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.