A SARS-like virus is spreading quickly. Here’s what you need to know.

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On December 31 last year, China announced the outbreak of a mysterious new coronavirus in the city of Wuhan involving a group of people who’d been exposed to animals at a local food market.

Flash-forward to Sunday: The number of cases of 2019-nCoV, as the virus is known, has leaped past 2,000, and there are sick people in at least 11 other countries.

That includes three confirmed cases in the United States.

The first, reported on January 21, involves a man in his 30s in Washington state. He tested positive for 2019-nCoV after being hospitalized with pneumonia, according to the Centers for Disease Control and Prevention (CDC). A second case, reported by CDC on January 24, involves a woman in her 60s who lives in Chicago. She returned from Wuhan on January 13, and is still in hospital. A third, reported January 25, concerns a person in Orange Country, California who also traveled from Wuhan recently. They’re in hospital and in “good condition.”

Nancy Messonnier, director of the CDC’s National Center for Immunization and Respiratory Diseases, said Friday that although the outbreak is a “very serious public health threat, the immediate risk to the US public is low at this time.”

Yet Tom Frieden, the former director of the CDC, told Vox he’s concerned by how infectious the virus is. “If the sustained human transmission and a high rate of severe illness are confirmed, then it clearly is an event of international concern.”

With information about 2019-nCoV — and its risk of spreading — evolving by the hour, we’ve answered basic questions about the outbreak here. We’ll be updating this story as more information becomes available.

1) Where did this coronavirus outbreak come from?

The outbreak was first reported to the World Health Organization by Chinese officials on December 31 in Wuhan, a city of 11 million in Hubei province. At that time, cases centered around Wuhan’s Huanan South China Seafood Market. Local health officials reported then that patients with the virus were “mainly business staff and purchasers” at the market, where vendors peddle seafood as well as animals such as birds and rabbits.

Scientists in China quickly ruled out known pathogens as the likely cause. The leading hypothesis then was that a yet-to-be-identified novel virus had spread to humans from one of the animals in the market.

A staff member screens arriving passengers with thermal scanners at Hankou railway station in Wuhan on January 21, 2020.

By January 9, the state broadcaster, China Central Television, reported a major discovery: a new virus, known as 2019-nCoV. Many of the people who had become ill tested positive for 2019-nCoV. Two days later, Chinese scientists shared the genetic sequence of the new virus, and the WHO applauded China’s efforts for sharing information readily. (This transparency was a contrast to the SARS outbreak of 2003, when China was heavily criticized for withholding information about the outbreak for too long. The virus eventually killed 774 people and infected more than 8,000.)

While Chinese authorities still believe the 2019-nCoV outbreak may have started at the market — with animals spreading the virus directly to humans — on January 20, they confirmed that human-to-human transmission is also occurring. By January 23, the WHO confirmed that there is “fourth-generation” spread of the virus in Wuhan, meaning there are cases where an individual has spread it to a second person, that second person to a third, and the third to a fourth. (Outside of Wuhan, there’s also evidence of second-generation cases.) Knowing that, “the scope of this outbreak expands massively,” Peter Daszak, the president of EcoHealth Alliance, a US global health research organization working in China, told Vox.

2) How many people are sick? How many have died?

As of January 26, at least 2,019 people have fallen ill. The vast majority of these patients (1,979) are in mainland China, where most (1,052) are concentrated in Hubei province, home to Wuhan. The remaining cases are spread across more than 30 Chinese provinces and cities — including Shanghai, Guangdong, Hong Kong, and Beijing. To get the latest numbers, check out this website from researchers at Johns Hopkins; it collects data from various sources: the WHO, the CDC, the National Health Commission of the People’s Republic of China (NHC), and two other sites. (The map is embedded below.)

Source: Johns Hopkins University Center for Systems Science and Engineering

Additional cases have turned up in travelers in 11 other countries, including the US, Thailand, Japan, Taiwan, Malaysia, Singapore, France, Australia, Nepal, Vietnam, and South Korea. A total of 56 people have died (all in mainland China).

But the real toll may be much higher.

UK-based researchers who have modeled the outbreak’s potential suggest there are likely many more cases — as many as 9,700, according to the upper end of their estimate. And the risk of rapid spread is heightened, as hundreds of millions start to travel this week for China’s Lunar New Year.

3) How is the world responding?

Chinese authorities first warned people to stop traveling in and out of Wuhan, and then said they would put the city under quarantine beginning January 23, suspending public transport within Wuhan, and canceling flights and trains leaving the city. It was an extraordinary move given that Wuhan has a population of 11 million, more people than New York City. By the evening of January 24, quarantine measures had expanded to 12 cities near Wuhan, effectively stifling the movement of 35 million people. By January 25, President Xi Jinping said the new virus posed a “grave” threat, and his government ordered travel agencies to suspend tours for Chinese citizens inside China and overseas. The same day, Hong Kong authorities declared a state of emergency over the virus, and decided to close schools for three weeks, while Beijing suspended all inter-province buses.

According to the New York Times, the government said the quarantine was needed to “effectively cut off the transmission of the virus, resolutely curb the spread of the epidemic, and ensure the safety and health of the people.” But the evidence for travel restrictions during outbreaks shows, counterintuitively, that they don’t actually do much to stop the spread of disease.

Health officials in China and the region are now scrambling to find cases through screening at transportation hubs and follow-up with the contacts of those infected with the virus — an effort that’s supported by the WHO.

Other countries at risk of imported cases have also implemented screening measures. In the US, the CDC — along with the Homeland Security Department’s Customs and Border Protection — began screening travelers from Wuhan for the virus at US airports on January 17, and all travelers from the city will be funneled through airports that are looking out for cases. On January 25, the US government announced it would be working to evacuate American diplomats and citizens from Wuhan.

4) What do we know about the cases in the US?

The third case, reported on January 25, concerns a person in Orange Country, California who also traveled from Wuhan recently. “The individual has now been transported to a local hospital and is in isolation in good condition,” according to the Orange County heath department, which didn’t provide any other patient details.

The second case, reported on January 24, involves a woman in her 60s who lives in Chicago. She returned from Wuhan on January 13, and didn’t have any symptoms while traveling. A few days after returning, when she started to feel ill, she called healthcare providers to report her symptoms and was admitted to hospital and placed in isolation. Right now, she’s in stable condition.

The CDC would not say which hospital she’s in at this time, but they did say she’s had “limited movement” outside of her house since coming home from China.

The first patient’s name also hasn’t been released, but we know he’s a resident of Snohomish County in Washington state. We also know he recently traveled to Wuhan, returning to Seattle on January 15, two days before airport screening started (though the man didn’t have symptoms yet). By January 19, he reached out to health care providers to report his symptoms and travel history. They suspected 2019-nCoV.

The man is being treated for mild pneumonia at Providence Regional Medical Center in Everett, Washington. He told officials he didn’t visit markets in Wuhan, or have any contact with sick people. The CDC and health officials in Washington are starting to do contact tracing to suss out if the man transmitted the virus to anyone else in the US. So far, no one he had contact with has fallen ill.

As of January 24, CDC officials said they’ve screened over 2,000 people coming into the US for the disease on 200 flights, and found zero cases. The agency also said it’s investigating 63 possible cases of coronavirus in 22 states. So far, two have been confirmed positive and 11 negative.

5) How does the new coronavirus spread?

We don’t yet know the exact way the virus is spreading, but we do know it’s part of a large family of viruses called coronaviruses, which mostly infect mammals, including bats. Coronaviruses attack the respiratory system, sometimes targeting the cells deep within the lungs. Only seven, including 2019-nCoV, SARS, and MERS, have evolved to infect humans.

According to the CDC, human coronaviruses are most commonly passed via:

Through coughing and sneezing
Close personal contact, including touching and shaking hands
Touching an object or surface with the virus on it, then touching your mouth, nose, or eyes before washing your hands
And rarely, through fecal contamination

As for how easily the new coronavirus spreads, that’s also unclear. According to a preliminary estimate from WHO, at the moment, each individual has transmitted the virus to an average of 1.4 to 2.5 others. That makes 2019-nCoV, based on what we know now, less contagious than SARS. But there are many different estimates floating around as the situation evolves — and some suggest 2019-nCoV is even more contagious.

We also know at least 15 health care workers have been infected. “It’s unusual to get health care worker infections in outbreaks,” explained Tom Inglesby, director of the Center for Health Security of the Johns Hopkins Bloomberg School of Public Health. “It’s always a sign of alarm when that happens,” he added, because it means the virus may be easily transmitted, even in settings where people are taking precautions.

6) What are the symptoms of the new coronavirus?

Two of the seven coronaviruses that infect humans, SARS and MERS, can cause severe pneumonia and even death. The rest lead to milder symptoms, like a common cold. Right now, it’s not clear where 2019-nCoV falls on that spectrum.

An early report, published in the Lancet, had a good overview of what’s known about symptoms so far — albeit, in a subset of the first 41 patients with confirmed 2019-nCoV in Wuhan. The most common symptoms in this group were fever, cough, muscle pain, and fatigue; less common were headache, diarrhea, and coughing up mucus or blood. A little more than half of the 41 patients had difficulty breathing, and 63 percent had low levels of white blood cells. As for the disease severity: 13 patients were admitted to an ICU and six died. By January 22, 68 percent of the patients had been discharged from hospital.

All patients had pneumonia and lung abnormalities on CT scans. (A quick refresher: Pneumonia is an infection in the lungs that can be caused by a variety of organisms — bacteria, fungi, viruses, even parasites. In this case, 2019-nCoV is causing the infection and leading the lungs’ air sacs to become inflamed and fill up with fluid or pus instead of air.)

A passenger walks past a notice about the coronavirus from Wuhan at Narita airport in Narita, Japan, on January 17, 2020.

In the Lancet group, most patients were men and over half didn’t have underlying diseases. The median age was 49.

So far, Chinese health officials have said the deaths are occurring in mostly older people with pre-existing health conditions.

It’s possible there are many more people with the virus who have very mild symptoms or who are asymptomatic, said Dr. Jeremy Farrar, director of the Wellcome Trust, a research charity focused on global health. If that’s the case, and there are thousands carrying 2019-nCoV while only a few people have died, this outbreak will look milder.

It’s also possible this virus winds up behaving like SARS, said Inglesby, which kills about one in 10 patients infected. With SARS, “We saw substantial spread in health care settings where health care workers became ill during care for patients,” for example. “This is a very serious outbreak with the potential for widespread transmission.”

7) What’s the likelihood this becomes a global public health emergency?

The potential for this virus to spread further is so great that the WHO gathered an expert committee on Wednesday and Thursday to decide whether the emergence of 2019-nCoV constitutes a global public health emergency, a rare designation the agency gives outbreaks that pose an international threat.

By the end of Thursday, WHO’s Director-General, Tedros Adhanom Ghebreyesus, announced the committee’s decision that the outbreak doesn’t yet constitute one, mainly because of the limited number of cases outside of China, and China’s efforts to control the spread of the disease.

“Make no mistake, this is an emergency in China. But it has not yet become a global health emergency,” Tedros explained. He also vowed to reconvene the emergency committee in the coming days to review new data. “At this time, there is no evidence of human-to-human transmission outside China, but that doesn’t mean it won’t happen,” Tedros added.

Formally, a public health emergency of international concern, or PHEIC — pronounced “fake” — is defined as “an extraordinary event which is determined to constitute a public health risk to other States through the international spread of disease and to potentially require a coordinated international response.” In reality, it’s a political tool the WHO uses to sound the alarm about a serious disease that has caught the world off guard and put people’s health in danger. It’s meant to draw countries’ immediate attention, to galvanize resources, and stop the disease from spreading further across borders.

World Health Organization Director General Tedros Adhanom Ghebreyesus at the WHO headquarters in Geneva on October 18, 2019.

A key consideration in declaring a PHEIC is whether the disease threat is dire enough to risk countries enacting travel and trade restrictions. Declarations can be devastating to local economies and are often associated with economic losses. So they’re not taken lightly. In fact, the WHO has only declared a public health emergency five times since the International Health Regulations, which govern global health emergency responses, were enacted in 2007.

The first time was in 2009, with the outbreak of the H1N1 swine flu pandemic. The second time was in May 2014, when polio seemed to surge again, threatening the eradication effort. The third time, in August 2014, came as the Ebola outbreak in West Africa was growing out of control. The fourth was related to Zika in 2016. And the fifth, in 2019, was another outbreak of Ebola that’s ongoing in the Democratic Republic of the Congo.

8) How worried should I be about a pandemic?

There are too many unknowns to say. We don’t know which animal carries this virus. We don’t know how exactly it spreads. We don’t know how easily it spreads among people, or how deadly and severe the virus is.

To put the cases and deaths so far into perspective, remember that seasonal flu kills between 250,000 and 650,000 people each year. “Right now, you’re probably more likely to be catching flu than you are to be getting coronavirus,” said Devi Sridhar, chair in global public health at the University of Edinburgh.

Even so, the fact that cases are already turning up in so many countries — mere weeks after this outbreak was first declared — suggests we should brace ourselves for an escalation. The WHO stopped short of recommending any travel or trade restrictions at this time, instead suggesting people take precautions — like making sure their hands are clean and that they don’t cough on others if they’re sick.

Messonnier of CDC said there are likely to be more cases in the coming days — in the US and beyond. Her agency is advising Americans to avoid non-essential travel to Wuhan. As for those traveling to other parts of China, officials suggest avoiding contact with sick people, and practicing good hand hygiene.

“This is a rapidly changing situation — abroad and domestically — and we’re still learning,” Messonnier said.

That doesn’t, however, mean American officials expect the US to be inundated with dozens of patients or that the virus will spread broadly within the country. It’s the major cities in and around China that are most at risk, according to a paper posted on January 14 in the Journal of Travel Medicine. The top travel destinations out of Wuhan — and most at risk of spread of the infection — include Bangkok, Hong Kong, Tokyo, and Taipei.