By Hillary Leung , Amy Gunia and Madeleine Carlisle
Updated: January 21, 2020 7:58 AM ET | Originally published: January 6, 2020

Officials in China have confirmed that a virus that originated in the central Chinese city of Wuhan has been transmitted by human-to-human contact, and that 15 health workers have been infected—a major development for the contagious illness.

On Tuesday, China’s National Health Commission said a total of 291 cases had been reported, with 72 new cases in Hubei province, where Wuhan is located. The virus has also spread to at cities across mainland China and to Taiwan, South Korea, Thailand and Japan. It is now responsible for six deaths.

Taiwan confirmed its first case Tuesday. The territory’s Centers for Disease Control reports that the patient is a 50-year-old woman who works in Wuhan. She was brought to the hospital by airport staff in Taiwan after she was found to exhibit symptoms of the virus upon arrival.

Yuen Kwok-yung, a microbiologist at the University of Hong Kong, called on residents of Hong Kong not to go to Wuhan unless absolutely necessary. “If you really need to, wear a mask and practice good hand hygiene if you go to crowded places or visit a hospital,” he said during a press conference. He also advised those who return back to Hong Kong from Wuhan to wear a mask for 14 days to avoid spreading the virus in case they caught it.

The new developments prompted the World Health Organization on Monday to call a meeting for Wednesday to determine whether the outbreak should be declared an international public health emergency.

As of Tuesday, Chinese authorities said the virus has been detected in six mainland Chinese cities outside Wuhan. Among the confirmed cases were 13 patients in four cities—Shenzhen, Zhuhai, Zhanjiang and Huizhou—in the southern province of Guangdong. Nine patients have been diagnosed with the virus in Shenzhen, a port city that borders Hong Kong where there have been 99 suspected cases, though none officially confirmed. Two cases have been reported in Shanghai, and five in Beijing.

But some experts are concerned that the actual numbers are far greater. A study by researchers at the MRC Centre for Global Infectious Disease Analysis at Imperial College London used population and international travel data to estimate that there were more than 1,700 cases in Wuhan alone as of Jan. 16—a time when Chinese authorities were reporting just 41.

The increase in the number of confirmed cases comes just days before the Chinese New Year holiday, raising fears that the virus could spread further as millions across China travel home to visit their families.

In response to the outbreak, outbound travelers at Wuhan’s Tianhe International Airport are having their temperatures checked before boarding planes, and those displaying fever symptoms are quarantined.

At the Wuhan Railway Station, employees have been told that they will soon start checking travelers’ temperatures, China Daily reported.

Cases of the virus have so far been reported in South Korean, Thailand and Japan, sparking concern of a possible regional outbreak. China is the world’s biggest market in outbound tourism, with nearly 150 million visits made by Chinese travelers in 2018.

Chinese President Xi Jinping said Monday the virus would be brought under control. “People’s lives and health should be given top priority and the spread of the outbreak should be resolutely curbed,” state TV quoted Xi as saying.

What do we know about the virus?

Medical experts in China identified the mysterious disease as a novel coronavirus earlier this month. Most coronaviruses result in mild symptoms, including upper-respiratory tract infections like the common cold, according to the U.S. Centers for Disease Control and Protection (CDC). But the classification of the disease puts it in the same family as SARS and the Middle East respiratory syndrome (MERS), both of which killed hundreds.

Chinese health officials had said earlier that there is no evidence of human-to-human transmission of the virus, which has been linked to a seafood market closed for sanitation since Jan. 1. The market also reportedly sold wild animals as game food.

However, late Monday, officials confirmed there had been cases of human-to-human transmission, and health workers had taken ill.

“Surgical mask” is now one of the top trending topics on Chinese social media platform Weibo, with users discussing which kinds of masks are best for preventing infections and others saying that they have ordered boxes of masks.

One user wrote: “I woke up reading about the sharp increase in confirmed cases, so I immediately placed an order of surgical masks on [e-commerce site] jd.com. Now, the source of the virus has not been confirmed, virus type unclear, transmission path unclear. I’m a little worried.”

Others reported that surgical masks were sold out when they visited local drug stores.

Arnold Monto, a professor in the School of Public Health at the University of Michigan, tells TIME that it may take some time to determine how the disease spreads.

“Something new may have unusual patterns for transmission,” he says.

Read more: The Wuhan Pneumonia Crisis Highlights the Danger in China’s Opaque Way of Doing Things

How serious is the novel coronavirus outbreak?

As of Tuesday, six people have died in Wuhan according to the city’s mayor Zhou Xianwang, and 258 patients in Wuhan have been diagnosed with the virus. The latest death was of an 89-year-old man who authorities said had pre-existing conditions including diabetes and high blood pressure.

As of Tuesday morning, 51 people in Wuhan were in serious condition, and 12 were in critical condition. Of the 15 medical workers, one is in critical condition.

In the southern province of Guangdong, where 13 patients across four cities are infected with the virus, four are in serious condition and two in critical condition.

Where has the virus spread to outside China?

South Korea confirmed its first case Monday. The 35-year-old Chinese woman arrived in Seoul from Wuhan on Sunday, according to South Korea’s Centers for Disease Control and Prevention.

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Thailand and Japan detected cases of the virus last week—authorities in both countries say that patients had visited Wuhan before falling ill.

According to Japanese health authorities, the patient, who is in his thirties, said that he had contact with a pneumonia patient while in Wuhan, but he did not visit the seafood market.

In Thailand, authorities detected a fever in a 61-year-old woman from Wuhan when she landed in the airport in Bangkok and hospitalized her the same day. Last week, Thailand health authorities announced that a second woman from Wuhan, who has been quarantined since she arrived in the country, also had the virus. Thai officials confirmed the patients were in “good critical condition.”

When asked if authorities are considering banning travel from Wuhan to Thailand, Suthat Chotanapan, the director of the Bureau of Risk Communication and Health Behavior Development for the Ministry of Public Health in Thailand, tells TIME in an email that, “We will follow the lead agencies such as WHO or U.S. CDC. At this moment there is still no travel notice that ban for travel to Wuhan. But please be reminded that there is a risk for this virus if you will be there.”

There are 99 unconfirmed cases of the virus in Hong Kong, where high demand for N95 masks made by 3M—which were commonly worn during the SARS outbreak—has caused stock to run out.

Authorities in Vietnam have also placed visitors suspected of having the disease under quarantine.

The first possible case of the virus was reported in Wuhan on Dec. 12, the city’s health bureau says.

How are authorities responding?

Governments are stepping up precautionary measures in the wake of the outbreak. Hong Kong, Singapore, Malaysia, South Korea and Thailand are also among the places where airport officials are screening passengers arriving from Wuhan.

The U.S. CDC said last week that airports across the U.S. will also screen people traveling from Wuhan for the virus, including at JFK International Airport, San Francisco International Airport and Los Angeles International Airport. The U.S. embassy issued a low-level alert for travel to Wuhan earlier this month.

On Tuesday, authorities in Australia announced that though the risk to Australians is low, that precaution will be taken at the Sydney airport where three direct flights from Wuhan arrive each week. All passengers coming from Wuhan will be screened and asked if they are experiencing symptoms such as fever, cough, breathlessness and sore throat.

The World Health Organization (WHO) says it is proposing studies on the coronavirus “to better understand transmission, risk factors, and where the virus is.”

Chinese authorities were criticized for trying to cover up the SARS epidemic, which started in southern China, but experts say they have learned from their mistakes.

“I think the public health authorities in China realized that that really was not the way to handle things, that things come out eventually, that response is best when it is handled promptly,” Monto, the University of Michigan public health professor, says.

While a number of cases have now been confirmed in other Chinese cities, little still is known about the novel disease. Experts say this may be due to ongoing development of testing for the new form of coronavirus.

“In the recognition of any new disease, there’s a problem in trying to be sure you’re identifying that particular disease when you don’t really have the diagnostic test,” Monto says. “When it’s a novel agent, developing a quick test takes awhile.”

Could this evolve into a large-scale outbreak like SARS?

Experts say the rapid developments of the past few days are worrying, but also point out that Chinese authorities are responding much more quickly to the virus this time than they did to SARS in 2003.

“They closed the seafood market on Jan. 7, shared the genome sequence on Jan. 10 and passed it onto WHO on Jan. 12,” David Hui, a respiratory medicine expert at the Chinese University of Hong Kong, says. “In terms of lab investigation, they have been highly efficient.”

According to Hui, the virulence of the disease appears to be on the low side. “I think the chances of the virus developing into an international outbreak is not very high,” he says, but adds that authorities need to locate the source of the virus as soon as possible to prevent a further spread.

“The sooner they find it, the easier it would be to control the outbreak,” he says.

Still, professor Gabriel Leung, the dean of the University of Hong Kong’s Faculty of Medicine, said at a press conference Tuesday: “[There is] a very strong sense of deja vu [with SARS], except the time scale has been compressed.”

He added: “Whereas you saw an unrecognized epidemic brewing for months since the end of 2002 up until the peak of it in March and April in Hong Kong, here you are talking about the same number, but the unit is weeks.”

Statistics modelling led by Leung suggests that there could be over 1,300 cases in Wuhan, in line with research by the London-based team that states the number of patients infected with the virus is significantly more than is being reported. Leung also says there could be one or two cases in Hong Kong, but insufficient data makes it difficult to make credible predictions.

How can you protect yourself?

As the number of cases continue to rise, health authorities are calling on the public to exercise increased caution. Wuhan’s health bureau has advised residents to seek medical attention immediately if they experience symptoms including fever or cough.

Public health officials in Hong Kong said residents should wear masks if they don’t feel well, or when visiting hospitals.

The Hong Kong Center for Health Protection also advised against contact with live animals or game meat.

Poon, the virologist at the University of Hong Kong, says it is best to avoid densely populated areas and maintain good personal hygiene, especially frequent hand washing.

But too little is known about the virus to give any definitive advice, he adds, such as whether citizens should wear a face mask in public places.

“We don’t know how efficient human-to-human transmission is, if it is caused for example by close contact or droplet transmission,” he says. We definitely need an answer for that. Without it, it’s hard to come up with practical preventive measures.”

— Aria Chen contributed to this report

Write to Hillary Leung at hillary.leung@time.com, Amy Gunia at amy.gunia@time.com and Madeleine Carlisle at madeleine.carlisle@time.com.

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