Thousands have been sickened by a new coronavirus in more than a dozen countries, including the US.

On December 31 last year, China announced a small outbreak of a new coronavirus in the city of Wuhan involving 27 cases.

Flash-forward to Wednesday: The number of cases of 2019-nCoV, as the virus is known, has leaped past 6,000, and there are sick people in more than a dozen other countries.

That includes five confirmed cases in the United States.

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.

AFP via Getty Images

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.)

By January 20, Chinese authorities 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.

But the picture of the beginnings of this outbreak is changing. A recent study, published on January 24 in The Lancet, suggests the outbreak may not have actually started at the market. The first known patient had no market exposure. And that person became ill on December 1, nearly two weeks earlier than Wuhan health authorities had suggested about the first case.

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

As of January 29, at least 6,057 people have fallen ill. The vast majority of these patients (5,970) are in mainland China, and most (3,554) 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 more than a dozen other countries, including the US, Canada, Thailand, Japan, Taiwan, Malaysia, Singapore, France, Australia, Nepal, Vietnam, Germany, and South Korea. A total of 132 people have died.

But the real toll may be much higher.

Hong Kong University infectious disease researchers have estimated that the virus may be spreading person-to-person in all of China’s major mainland cities, and that there are already 44,000 cases in Wuhan alone. 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 have been traveling 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 closed 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 extraordinary 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. The US government evacuated American diplomats and citizens from Wuhan. Its also advising Americans to avoid non-essential travel to China. Hong Kong has suspended ferry and train links with mainland China, and cancelled flights. Meanwhile, several airlines have begun cancelling flights to China — including Air Canada, British Airways, and United.

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

Right now there are five confirmed cases in the US, according to the CDC. All involved travelers from Wuhan:

The first patient’s name hasn’t been released, but we know he’s a resident of Snohomish County in Washington state. We also know he’s in his 30s and 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.

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 health care providers to report her symptoms and was admitted to a hospital and placed in isolation. She’s currently in stable condition.

The third, fourth, and fifth cases, reported over the weekend, concerned individuals in Orange County and LA County, California, and Maricopa County, Arizona, respectively. They had all recently traveled to Wuhan, and they’re receiving care in local hospitals.

As of January 25, 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 100 possible cases of coronavirus in 26 states.

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 the 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.

For the moment, the WHO estimates the virus’s incubation period — before symptoms start to show — is between two and 10 days, but it’s not yet known whether the virus can spread during that time.

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. At the moment, it’s not clear where 2019-nCoV falls on that spectrum.

An early report, published in the Lancet, provided a good overview of what’s known about symptoms so far — 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 the 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.

Tomohiro Ohsumi/Getty Images

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

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 last week 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, the 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.

Fabrice Coffrini/AFP via Getty Images

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.

Messonnier of the CDC said there are likely to be more cases in the coming days — in the US and beyond. “This is a rapidly changing situation — abroad and domestically — and we’re still learning,” Messonnier added.

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.

Listen to Today, Explained

A SARS-like virus has sickened thousands in China, and made its way to the United States. Vox’s Julia Belluz explains what’s known and what’s next.

Subscribe to Today, Explained wherever you get your podcasts, including: Apple Podcasts, Google Podcasts, Spotify, Stitcher, and ART19.