A respiratory virus that is so new it does not yet have a name has spread quickly from Wuhan, China, leaving many experts to fear what may become a pandemic of pneumonialike illness.
So far, the virus - a coronavirus whose scientific designation is 2019-nCoV - has spread to at least 23 countries, infecting more than 17,000 people and killing at least 425. Most of the cases, so far, have been in China.
But the Wuhan coronavirus may be highly transmissible, as contagious as seasonal influenza, and the death rate is still unknown.
The World Health Organisation (WHO) has estimated that one in every 50 infected individuals will die, and medical reports suggest the new virus can be transmitted even before an infected individual displays symptoms - a combination of factors that, if accurate, will make it nearly impossible to contain.
1in 50new coronavirus-infected individuals will die, estimates WHO.
Last week the WHO declared a global public health emergency, and made plans to dispatch experts to China to help with the investigation and containment.
The United States also declared a public health emergency and has offered assistance, and is barring entry by most foreign nationals who have visited China recently. American travellers returning from trips in the Wuhan/Hubei region will be quarantined for two weeks.
The outbreak appears to have begun at a seafood and meat market in Wuhan. Since then it has spread to other parts of China, and turned up in other countries in Asia and in Australia, Europe and North America.
Chinese authorities cordoned off Wuhan days before the Lunar New Year holiday, suspending transportation in and out of the provincial seat of Hubei and other affected cities, closing in an estimated 50 million people.
Federal health officials are urging Americans to avoid all travel to China, and have quarantined 195 Americans evacuated from Wuhan for 14 days.
"This is a very serious public health situation, and the CDC and federal government have and will continue to take aggressive action to protect the public," said Dr Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases. The quarantine of a planeload of American evacuees in Southern California was an "unprecedented action," she added, provoked by an "unprecedented threat".
As of Monday, 11 cases have been confirmed in the United States, most of them in patients who had travelled to China. One patient who had not travelled was apparently infected by his wife when she returned from China. More than 100 other patients in the United States are being tested.
Experts cannot yet say who is at greatest risk for developing severe or life-threatening disease, and what factors may be protective against the illness.
Here's what we have learned so far about the virus and the outbreak.
What is a coronavirus?
Coronaviruses are named for the spikes that protrude from their membranes, which resemble a crown or the sun's corona. They can infect both animals and people, and can cause illnesses of the respiratory tract. The infections can range from the common cold to dangerous conditions like severe acute respiratory syndrome, or SARS, which sickened thousands of people around the world - and killed nearly 800 - during an outbreak in 2003. Middle East respiratory syndrome, or MERS, first reported in Saudi Arabia in 2012, is also caused by a coronavirus.
How dangerous is it?
It is hard to accurately assess the lethality of a new virus, and some scientists said initially that the new virus appeared to be less severe than other coronaviruses, like SARS or MERS.
But recent news reports indicate the Chinese authorities muzzled doctors who were first to recognize the new infection, and they may not have been fully transparent about the number of infections and deaths even after the alarm was raised.
It is also not clear whether cases and deaths in China are being carefully tracked and reported. Diagnostic kits and other health resources have been in short supply in the affected regions.
Last week, the WHO said that while the majority of patients with the new coronavirus have mild disease, 20 per cent develop severe illness, including pneumonia, respiratory failure and sometimes death.
The WHO estimated the death rate among those infected at 2 per cent. That's as high as the death rate of the pandemic Spanish flu in 1918.
20,438people are known to have been infected by the new coronavirus.
Scientists don't know who is most susceptible to the new coronavirus. Does it pose a risk to young healthy people? Or, like seasonal flu, is it primarily a threat to the frail and elderly, and those with underlying health problems?
"Whenever a new virus comes out, it takes a while to learn about it," said Dr Julie Vaishampayan, chairwoman of the public health committee of the Infectious Diseases Society of America. Right now, she said, "The information about coronavirus is changing rapidly."
How many people have been infected, and how many have died?
At least 20,438 people are known to have been infected, and at least 425 have died. Most of the infected lived in Wuhan or had recently travelled there.
Epidemics caused by other members of the coronavirus family, SARS and MERS, have had high death rates: 10% for SARS, and about 35 per cent for MERS.
Influenza kills more people every year, but its mortality rate is only about 0.1 per cent. The number of deaths is high because so many people become infected.
Widespread coronavirus epidemics have the potential to take a heavy toll, so the health authorities scramble to stamp them out.
A major concern is that with both SARS and MERS, a few patients inexplicably became "superspreaders" who infected huge numbers of people.
Health care workers are particularly vulnerable. At a hospital in Seoul, South Korea, in 2015, one man with MERS transmitted it to 82 patients.
How is it transmitted?
Most respiratory viruses are transmitted through coughing and sneezing, but troubling new evidence suggests people who are infected with the new virus may be spreading it before they even know that they have any symptoms.
Though Chinese authorities played down the likelihood of human-to-human transmission at first, it has now become clear there is significant and sustained transmission from person to person.
In Germany, a 35-year-old businessman was infected by a colleague visiting from Shanghai. She was healthy during her trip to Germany, and only fell ill during her flight home. Three other employees of the man's company were also infected; two of them had had no contact with the visitor.
That's "bad news," said Dr. William Schaffner, an expert in infectious diseases at Vanderbilt University Medical Center.
When people don't know they are infected, "they're up and about, going to work or the gym or to religious services, and breathing on or near other people," Schaffner said.
"Obviously sick people constrain themselves, and you have fewer opportunities for the virus to spread from one person to another."
Where did the new coronavirus outbreak start?
On January 8, The New York Times reported that Chinese researchers had identified a new coronavirus as the pathogen behind a mysterious illness that had sickened 59 people in Wuhan.
The cases were linked to a market that sold live fish, animals and birds. The market was later shut down and disinfected.
Past outbreaks of similar illnesses, including SARS, also are believed to have emerged from live animal markets. The coronavirus that causes MERS is transmitted to humans by camels.
The animal that was the source of the new coronavirus is still not known, and the destruction of the Wuhan meat market has made the question nearly impossible to investigate.
Bats are considered a possible source, because they have evolved to coexist with many viruses, including coronaviruses. But it's very possible the virus was transmitted from bats to an intermediate animal, and then to humans.
What symptoms should I look out for?
Symptoms of this virus include fever, severe cough and difficulty breathing or shortness of breath. The illness causes lung lesions and pneumonia. Milder cases may resemble the flu or a bad cold, making detection difficult.
Patients may exhibit other symptoms too, such as gastrointestinal problems or diarrhea. The incubation period - the time from exposure to the onset of symptoms - is believed to be anywhere from a few days to two weeks.
If you have a fever or cough and recently visited China, or spent time with someone who did, see your health care provider. Call first, so they can prepare for your visit and take steps to protect other patients and staff from potential exposure.
Is there a test for the virus? What is the treatment?
There is a diagnostic test that can tell if you are infected. It was developed by the Centers for Disease Control and Prevention, based on genetic information about the virus provided by Chinese authorities.
Right now tests are done by the CDC in Atlanta, but the agency plans to share the test with states and laboratories as soon as it is refined.
The main treatment for coronavirus is supportive care, including making sure the patient is getting enough oxygen, and using a ventilator to push air into the lungs if necessary, Vaishampayan said. Patients should rest and drink plenty of fluids "while the immune system does its job and heals itself," she said.
No drugs have been approved for any coronavirus diseases, including the Wuhan coronavirus, though an antiviral medication called remdesivir appears to be effective in animals and was used to treat a patient in Washington state.
Chinese officials are also experimenting with at least one other HIV drug to treat the infection.
How long will it take to develop a vaccine?
A coronavirus vaccine is still months away - and perhaps years. While new technology, advancements in genomics and improved global coordination have allowed researchers to move at unprecedented speed, vaccine development remains an expensive and risky process.
With each new outbreak, scientists typically have to start from scratch. After the SARS outbreak in 2003, it took researchers about 20 months to get a vaccine ready for human trials. (The vaccine was never needed, because the disease was eventually contained.)
By the time of the Zika outbreak in 2015, researchers had brought the vaccine development timeline down to six months.
Now, they hope that work from past outbreaks will help cut the timeline further. Scientists at the National Institutes of Health and several companies are working on vaccine candidates.
Dr Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said a preliminary clinical trial may get off the ground in as little as three months. But researchers would still need to conduct extensive testing to prove a vaccine is safe and effective.
Even under the best circumstances, it could take at least a year - maybe longer - for a vaccine to be available.
Should I wear a surgical mask to protect myself?
If you have a respiratory infection, wearing a mask helps protect the people around you from illness by reducing the risk that you will spread the infection. And wearing a surgical mask may somewhat protect you from infection in a crowd if there is an outbreak.
But, generally, surgical masks are not closefitting enough to filter all the air you are breathing in and N95 respirators are extremely uncomfortable.
Experts recommend washing your hands frequently throughout the day. Avoid touching your face, and maintain a distance from anyone who is coughing or sneezing.
At the moment, the risk of infection with the new coronavirus in the United States - where there are 11 confirmed cases so far - "is way too low for the general public to start wearing a face mask," Rabinowitz said.
But, he added, "if you have symptoms of a respiratory illness, wearing a mask reduces the risk of infecting others."
I have a trip planned to China. Should I go?
Not if you can avoid it. The State Department has warned Americans not to go to China unless it is absolutely essential.
If travellers must go, the CDC urges enhanced precautions: avoiding contact with anyone who is sick, as well as with animals and the markets in which they are sold; and refraining from eating raw or undercooked meats.
Anyone who is older or has an underlying health condition, which increases susceptibility to infections, should consult with a health provider before making a trip. Access to medical care in China may be difficult, given the outbreak, and federal officials warned that new travel restrictions could be imposed on return, including quarantine.
What are the health authorities doing to contain the virus?
China has taken drastic actions to bar people from leaving affected areas, though 5 million people departed Wuhan alone before the restrictions went into effect.
Residents have been told to wear masks in public to prevent the spread of the disease. Two new hospitals for coronavirus patients are being erected; the first opened on Monday.
Governments around the world have been screening incoming passengers from China for signs of illness, and some have gone further, barring entry to people from China. Russia and Mongolia have closed most of their borders with China. Australia said it will evacuate citizens from Wuhan and quarantine them for 14 days on Christmas Island.
CDC teams are also providing assistance with state health investigations of infections, including contact-tracing, which means making sure anyone who may have been in contact with an infected individual is warned of the exposure and their health is monitored.
The work will enhance the understanding of the virus and how to prevent its spread. CDC teams have also offered to send public health experts to China to help with the investigation and containment efforts.