WUHAN, China — The mysterious coronavirus that has killed at least four people and sickened more than 200 in China is capable of spreading from person to person, a prominent Chinese scientist said on Monday, adding to fears of a broader epidemic.
The disclosure increased pressure on the Chinese government to contain a growing public health crisis, just as China enters its busiest travel season of the year. On Tuesday, the authorities confirmed a fourth death from the illness in the central Chinese city of Wuhan.
The authorities had previously said the deadly virus seemed capable of spreading only from animals to humans in most cases, tracing the outbreak to a market in Wuhan.
But in recent days at least two people have become infected with the pneumonialike virus even though they live hundreds of miles from Wuhan, experts said on Monday, suggesting that the illness is spreading from person to person.
“Now we can say it is certain that it is a human-to-human transmission phenomenon,” Dr. Zhong Nanshan, a scientist who is leading a government-appointed expert panel on the outbreak, said in an interview on state-run television on Monday.
The World Health Organization announced Monday that it was convening an emergency meeting on Wednesday “to ascertain whether the outbreak constitutes a public health emergency of international concern, and what recommendations should be made to manage it.”
Millions of Chinese are traveling this week for the Lunar New Year holiday, adding to fears that the virus could spread swiftly and on a broader scale. Experts said the severity of the outbreak would now depend on how many people, on average, a person with virus could infect.
“There are now sufficient cases that it’s not going to die out by chance,” said Neil Ferguson, a public health expert at Imperial College London who has studied the new virus. “The real question now is, how efficiently can this virus spread from person to person?”
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.
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.
The authorities have started detecting the country’s first cases outside of Wuhan. Five new cases were reported in Beijing, 14 in Guangdong Province, and two in Shanghai. The total number of cases climbed to at least 219.
The World Health Organization said the spike in reported cases was the result of increased searching and testing for respiratory illnesses.
The virus now appears to spread between humans.
Researchers in China identified the mysterious pneumonialike illness in early January as a new coronavirus.
Experts initially said that it did not appear to be readily spread by humans, but on Monday Dr. Zhong, the scientist appointed by the government to study the outbreak, said that in Guangdong, two individuals appeared to have been infected by relatives who had traveled to Wuhan.
Dr. Zhong said the virus could be present in particles of saliva. In one case, he said, a patient appears to have infected 14 medical workers.
Coronaviruses are named for 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. Dr. Zhong was China’s leading SARS expert during that outbreak.
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.
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 serves 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 on anyone believed to have come in contact with the woman in the plane, she said.
Officials have implemented precautionary measures.
In Hong Kong, the government initiated a disease response plan after the first cases were reported in Wuhan. Dozens of people were hospitalized after returning to Hong Kong from mainland China, but none were found to have been infected with the new virus.
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 said it would also crack down on the illegal trading of wild animals.
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.
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.
Javier C. Hernández reported from Wuhan and Austin Ramzy reported from Hong Kong. Choe Sang-Hun contributed reporting from Seoul, South Korea. Elsie Chen contributed research from Wuhan, and Albee Zhang contributed research from Beijing.