As China continues to implement emergency measures to contain a novel coronavirus, U.S. health officials announced that a second travel-related case has been confirmed in the U.S. The patient is a female Chicago resident in her 60s who returned from Wuhan, China on Jan. 13.

Chinese authorities have shut down all public transport in and transportation routes into Wuhan — the central city of 11 million people where the coronavirus outbreak originated. The government announced that all outbound travel would also be suspended as of Thursday morning and Wuhan has already started closing highway exits leaving the city.

At least 14 cities in Hubei Province, where Wuhan is located, have enacted transportation restrictions to prevent the spread of the virus, which has so far killed 41 people and led to nearly 1,300 confirmed cases nationwide, according to Chinese state media. Huanggang has suspended all public road services. Xiantao and Chibi have at least partially suspended public transport services and set up temperature screening checkpoints at highway entries. Ezhou has announced that it will close its train stations.

Local authorities in Wuhan, where museums and theaters are also closed, had previously specified that all people in public places must wear masks. A 24-hour hotline has been opened to help locals receive medical supplies and protective equipment and an order to suspend tax-hailing services was scheduled to go in effect Friday at noon.

China’s finance ministry has allocated $144 million to Hubei Province to fight the disease.

The World Health Organization decided not to declare the outbreak a “public health emergency of international concern” after convening an emergency committee that met on Wednesday and Thursday, saying that it was still “too early” to do so.

WHO Director General Tedros Adhanom Ghebreyesus said he wanted to reiterate that the decision “ should not be taken as a sign that WHO does not think the situation is serious.”

“Make no mistake. This is an emergency in China but it has not yet become a global health emergency. It may yet become one,” Tedros said.

The committee said that it was ready to reconvene in about 10 days time or earlier if Tedros found it necessary and would continue to support efforts to investigate the animal source of the virus, the extent of human-to-human transmission and screening efforts in other Chinese provinces.

Asked about the strict transportation measures the Wuhan government has taken to contain the virus, Tedros said on Thursday that the WHO’s role is to provide public health guidance and recommendations to countries but that “China is a sovereign nation with the autonomy to take steps it believes are in its interest.”

“China has taken measures which it believes will be effective but we hope from our side that they are both effective and short in duration,” Tedros said.

The virus was first detected as a form of viral pneumonia centered on a seafood market in Wuhan on Dec. 12. Many of the first reported cases were people who worked at the market, which also sold wild animal meat. Officials closed down the market and identified the illness as a novel coronavirus, named 2019-nCoV. The Wuhan government announced Wednesday that The largest number of cases are in Wuhan, but it has also spread to over a dozen mainland cities.

The illness, which is in the same family as severe acute respiratory syndrome (SARS), spread to the United States on Tuesday, with a Washington State resident testing positive after a trip to China, according to the U.S. Centers for Disease Control and Prevention (CDC).

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. Some 450 million Chinese were expected to travel across the country and around the world for the holiday. An estimated 7 million Chinese citizens traveled internationally for the holiday in 2019, according to government figures.

The local government in Beijing has also cancelled large-scale Chinese New Year events. Beijing’s Forbidden City is also set to close from Saturday.

The sudden spike in numbers has created fears that this could become a repeat of SARS, which killed nearly 800 people and infected more than 8,000 after beginning in Southern China in 2002. The Chinese government was accused of covering up SARS as it spread through mainland China and into neighboring Hong Kong. Bustling cities emptied as people stayed home to avoid contact with the public, and financial markets were devastated globally.

The first U.S. patient is a resident of Snohomish County in his 30s, who arrived in Seattle on Jan. 15. He is currently being treated at Providence Regional Medical Center in Everett, Wash.

U.S. public health officials are working to “identify and contact all those who may have come in contact with the infected traveler,” the Washington State Department of Health said in a statement.

How many people have been infected in China?

As of Friday, Chinese state media reported 1,287 confirmed cases of the virus nationwide, resulting in 41 deaths. Most have been in Hubei Province. A 36-year-old man in Wuhan who died from the disease is the outbreak’s youngest victim.

Among the confirmed cases were at least 17 patients in four cities—Shenzhen, Zhuhai, Zhanjiang and Huizhou—in the southern province of Guangdong. At least 10 patients have been diagnosed with the virus in Shenzhen, a port city that borders Hong Kong where there have been 118 suspected cases, and two officially confirmed, according to local media. Thirty-four cases have been reported in Beijing, and at least nine in Shanghai.

In the southern province of Guangdong, at least 26 patients across seven cities are infected with the virus.

Where has the virus spread to outside China?

French Health Minister Agnes Buzyn said Friday that two cases of coronavirus have been detected in France, marking the illness’ first spread into Europe. A statement from the Health Ministry later in the day confirmed a third. All three cases—one in Bordeaux, and two in Paris—are travel-related.

On Wednesday, Hong Kong officials confirmed the first two cases of the coronavirus after saying for weeks that more than 100 patients were only suspected of having the disease.

Cases have also been confirmed in Saudi Arabia, Singapore, Vietnam, South Korea, Thailand, Japan, Macau and Taiwan—along with the U.S. On Thursday, Singapore confirmed its first case of the virus and Vietnam confirmed it’s first two cases, too. India’s health ministry said Thursday that they detected no cases of the virus after testing just under 13,000 passengers. However, an Indian nurse working in Saudi Arabia tested positive.

On Wednesday, Mexico’s president said that authorities were looking at two possible cases of the virus in the country, noting that one had been ruled out and another was still being monitored. Scotland, Australia and the Philippines were also looking into potential infections within their borders.

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.

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.”

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

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.

Any human-to-human transmission appears to be limited to families and health workers caring for infected patients, Tedros, Director General of the WHO, said at Thursday’s press conference by WHO. “At this time, there is no evidence of human-to-human transmission outside China but that doesn’t mean it won’t happen,” Tedros said. Most of those who died had underlying health conditions that involved weakened immune systems, like hypertension, diabetes or cardiovascular disease, he added.

There is still a lot experts don’t know about the virus. “We don’t know the source of this virus, we don’t understand how easily it spreads & we don’t fully understand its clinical features or severity,” Tedros said, noting that the organization was working to “fill the gaps in our knowledge as quickly as possible.”

Investigations to identify an animal source of the disease are underway in China and Wuhan, said Dr. Maria Van Kerkhove, head of emerging diseases and zoonosis at WHO, said Tuesday’s press conference.

“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

What do we know about the cases of the virus in the U.S.?

The CDC announced Friday that a female Chicago resident has been diagnosed with the coronavirus after returning from a trip to Wuhan on Jan. 13. The women, who Illinois health officials said is in her 60s, did not show symptoms while traveling, and called her doctor after beginning to feel ill days later—a decision praised by health officials, who urged others who feel sick and may be at risk to proactively seek care.

The woman’s doctor provided her with a mask and referred her to a hospital with infection control capabilities. Upon further isolated evaluation at the hospital and testing from the CDC, the woman was confirmed to have the coronavirus. She is stable and in good condition, health authorities said.

Illinois health officials said the woman had limited close contact with people outside her home after returning from Wuhan, and had not taken public transportation or gone to public events. Risk to others in the area is low, they said, and none of the woman’s close contacts have reported getting sick.

Her diagnosis follows that of a male resident of Snohomish County, Wash., who had returned to Seattle’s International Airport on Jan. 15 from a trip to Wuhan. Authorities said that the patient is in “good condition” but is hospitalized “out of an abundance of precaution.”

After returning from his trip, the man sought out care from a medical provider in Washington, according to state and federal health officials. “This was a very astute gentleman,” said Scott Lindquist, Washington state’s epidemiologist, who noted that the man had researched and shared information about the virus with his provider.

Health care professionals suspected that the man had been infected with the coronavirus based on his travel history and symptoms and alerted public health officials who later confirmed this was the case.

The CDC said the agency had already been preparing for the introduction of the virus into the U.S. “for weeks” and had told clinicians to be vigilant about patients reporting “respiratory symptoms and a history of travel to Wuhan” as early as Jan. 8.

CDC officials said on Friday that they expect to see more cases in the U.S., mainly among people who returned from Wuhan before the city implemented travel restrictions. The virus has an incubation period of about two weeks, CDC officials said, so patients who did not have symptoms while traveling may develop them later.

Travel restrictions in Wuhan should dramatically limit the number of people coming into the U.S. with coronavirus, CDC officials said. Nonetheless, five U.S. airports—New York City’s John F. Kennedy International Airport, Los Angeles International Airport, San Francisco International Airport, Chicago O’Hare International Airport and Hartsfield-Jackson Atlanta International Airport—continue to screen for signs of the disease. So far, about 2,000 people on 200 flights have been screened at U.S. airports.

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.

In addition to domestic airport screenings, the U.S. embassy issued a low-level alert for travel to Wuhan earlier this month. The CDC urged people to limit non-essential travel to Wuhan.

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.

Leo Poon, a virologist and SARS expert 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 in Hong Kong and Madeleine Carlisle and Jamie Ducharme in New York contributed to this report

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Write to Hillary Leung at hillary.leung@time.com, Amy Gunia at amy.gunia@time.com and Sanya Mansoor at sanya.mansoor@time.com.