An outbreak of a never-before-seen coronavirus in the Chinese city of Wuhan dramatically worsened over the last few days—the case count has more than tripled, cases have appeared in new cities, and authorities have confirmed that the virus is spreading person to person.

The World Health Organization announced Monday that it will convene an emergency meeting on Wednesday, January 22, to assess the outbreak and how best to manage it

On Saturday, January 18, the Wuhan Municipal Health Commission reported 136 newly identified cases of the viral pneumonia and one additional death. On Tuesday, January 21 (local time 4:18am), the commission reported another death. That brings Wuhan’s totals to 198 cases and four deaths. Just one day earlier, on January 17, the health commission had reported just 62 cases and two deaths.

The outbreak began in December there and has been linked to a live-animal market in Wuhan called the South China Seafood Wholesale Market. Researchers raced to identify the virus behind the outbreak, confirming quickly that the culprit is a never-before-seen coronavirus—a relative of the virus that caused the SARS (Severe Acute Respiratory Syndrome) outbreak in 2003. Like SARS, experts suspect that the new coronavirus leapt from animals at the market to humans, sparking the outbreak. Many strains of coronavirus circulate in animals and humans, causing mild to severe diseases.

But until this weekend, there were reasons to be optimistic that the new outbreak could be easily managed. Officials in Wuhan quickly shut down the live animal market January 1 for decontamination. On January 11, the health commission reported 41 confirmed cases, most of which had clear ties to the market, and there was no clear evidence that the virus was spreading from human to human. No medical staff had been infected, and there had been no sign of new cases since January 3.

That has all changed. As the case tally shot up over the last few days, officials say that there is clear evidence that the virus is spreading from person to person, and at least 14 medical staff members contracted the virus.

Booming bug

“Now we can say it is certain that it is a human-to-human transmission phenomenon,” SARS expert Zhong Nanshan said on state-run television on Monday, according to The New York Times. Zhong is leading a government-appointed expert panel on the outbreak.

Zhong elaborated that there was at least one case of human-to-human transmission in Wuhan, a city in central China, and two cases in families in the southern Guangdong province that borders Hong Kong. He also said that in one case, a single patient spread the virus to at least 14 medical staff members. He referred to such a patient as a “super-spreader,” according to South China Morning Post , and called these patients key to controlling the outbreak.

“We expect the number of infected cases will increase over the Lunar New Year travel period, and we need to prevent the emergence of a super-spreader of the virus,” Zhong said. Millions are expected to travel during the holiday period.

So far, officials have found cases of the virus in travelers to Thailand, South Korea, and Japan.

Besides Wuhan and Guangdong in mainland China, Chinese health officials have now reported cases in Beijing, Shanghai, and Shenzhen. The total number of cases overall in China has climbed to 218, according to state-run media.

Hong Kong has reported many suspected cases, but none have been confirmed to be linked to the outbreak. The region was hard hit by the 2003 SARS outbreak, which sickened over 8,000 people worldwide and killed 774.

Officials in Hong Kong and many other places have stepped up monitoring and are screening travelers. The US Centers for Disease Control and Prevention has already announced that it is closely monitoring the outbreak and will screen travelers from Wuhan.

The main symptoms of the virus are fever, cough, chest tightness, and difficulty breathing.