The WHO designating the outbreak a global health emergency would help countries coordinate their responses. In the meantime, it remains a scary time for people in China and beyond as the virus and fears of contamination spread.

On Tuesday, the United States confirmed its first case: A man who flew from China to Washington state is in stable condition. On Friday, U.S. authorities reported a second case of an infected woman, a Chicago resident, and France confirmed its first three patients as well. Travel bans in central China have left tens of millions of Chinese people effectively on lock down.

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As the news continues to develop, keep calm and catch up on what is known so far.

What is a coronavirus?

According to the WHO, coronaviruses are a large family of viruses that range from the common cold to much more serious diseases. These diseases can infect both humans and animals. The strain spreading in China is related to two other coronaviruses that have caused major outbreaks in recent years: Middle East respiratory syndrome, also known as MERS, and severe acute respiratory syndrome, or SARS.

Symptoms of a coronavirus infection range from respiratory problems, difficulties breathing, fever and cough, to the much more severe cases of pneumonia, kidney failure, acute respiratory syndrome (when fluid builds up in the lungs) and death. The elderly, young and those with an already weakened immune system are at a higher risk of developing severe lower-respiratory tract diseases, such as bronchitis and pneumonia, according to the Centers for Disease Control and Prevention.

Health officials haven’t identified this latest strain in humans before. That’s why, for now, it has the generic name of “a novel coronavirus” while they investigate. As it’s a virus, antibiotics won’t work in treating it.

How does it spread?

In rare cases, coronaviruses can spread from animals, such as camels and bats, to humans. (Household pets are not a threat.) Health officials report that’s what they believe has happened here. In even rarer cases, that same virus can then start to spread from human to human. That’s what happened with MERS and SARS.

WHO officials are investigating suspected cases of that now. In a worrying development, Chinese officials said Monday that they have documented cases of human-to-human transmission, meaning it can spread to humans through airborne droplets. The exact ways its spread and the incubation period are still under investigation.

Chinese health authorities said they first detected the new strain of the virus Dec. 31 in Wuhan, a city in central China. They initially linked it to a dirty food market where seafood and mammals were sold for human consumption. Officials closed the market the next day. What probably happened, scientists said, is that people ate something infected with the virus or touched something and then became infected.

The next set of patients are those who reported that they did not come in contact with that market but had gone to other markets, or had contact with others in Wuhan. Chinese officials have also documented patients and health-care workers who had no contact with Wuhan.

In cases of human-to-human transmission, the disease can spread through coughing and sneezing, personal contact with an infected person, touching an infected surface and then the mouth, nose or eyes, and, in rare cases, through fecal contamination.

How do you protect against it?

To protect against infection, the CDC recommends basic hygiene techniques for respiratory viruses such as constantly washing hands, staying hydrated, avoiding contact with one’s face or anyone who’s sick, sanitizing surfaces, and coughing into one’s arm or a tissue. If there’s a fear of animal transmission, CDC officials urge people to wash hands after contact with animals and thoroughly cook any meat before consumption.

What do we know about how new cases have spread?

One challenge to investigating — and stopping — the virus: Public health experts say that Chinese authorities have not provided full information about how the disease is spreading.

To control the outbreak, it’s critical to know whether cases being found in other cities are all related to Wuhan. If the disease has been circulating independently in other parts of the country, that information will not only affect how China acts to contain it, but how other public health agencies in the world seek to prevent its spread, said Tom Inglesby, director of the Johns Hopkins Center for Health Security.

(Laris Karklis)

The Post’s Anna Fifield has reported from Beijing on cases of people dying with pneumonia-like symptoms but not being included in the death toll, suggesting “that the coronavirus could be far more prevalent than Chinese health authorities have acknowledged.”

Most people sickened and killed by the virus have been elderly, had preexisting health conditions, and lived in Hubei Provence -- specifically its capital, Wuhan.

But Chinese health authorities announced that a 36-year-old man from Wuhan died Thursday: He had no chronic diseases or other previous health problems and had been treated with anti-virus medications since checking into the hospital on Jan. 9.

Where has it spread?

As of Sunday in China, officials said that at least 56 people have died of the virus and more than 1,900 have been confirmed infected. That number is way up from the more than 70 infections reported on the previous Monday. Experts expect the number to keep rising, amid claims that China has been underreporting cases.

So far, most cases have been in Wuhan, though there are confirmed cases all across China.

Officials in Thailand and Japan were the first outside of China to reports cases of infected travelers from Wuhan on Jan. 13 and 15.

“These cases did not report visiting the large seafood and animal market to which many cases in China have been linked,” the CDC reported last week, a factor that initially raised further concerns among health officials of human-to-human contamination.

Australia and the Philippines are investigating suspected cases. By Wednesday, Hong Kong and Macao confirmed their first infected patients, joining South Korea, Vietnam, Singapore, Nepal, France and Taiwan, which have each reported cases of the virus.

What’s being done to stop it in China?

On Tuesday, Chinese health authorities initially imposed a quasi-quarantine on Wuhan, limiting travel to the city, which is home to 11 million people. They did so in part because the upcoming Lunar New Year is a time when people often travel to their hometowns. That’s also why Chinese authorities may be hesitant to impose a total travel ban; they are reluctant to entirely disrupt the holiday, which is also a time of heavy commerce important to China’s economy.

Just two days later, however, Chinese authorities announced a more extreme step: Starting 10 a.m. Thursday morning there, they would ban all outbound travel from Wuhan, which remains the center of the outbreak.

Also on Thursday, Chinese authorities announced the closure of all large-scale Lunar New Year events in Beijing. One of China’s top tourist attractions, the Forbidden City in Beijing, said it would close “to avoid cross-infection caused by the gathering of people.”

The WHO has praised China for imposing the quarantine in an effort to contain the disease. But other public health experts have questioned the effectiveness and warned that there could be negative repercussions.

“In the eyes of some public health experts, the quarantine could also cost time trying to contain the virus,” reported The Post’s Marisa Iati and Reis Thebault. “Jennifer Nuzzo, a senior scholar at the Johns Hopkins Center for Health Security, said that people fleeing Wuhan to escape the restrictions make it hard for authorities to track where the virus may have spread. People who experience symptoms may also hesitate to come forward because of the government’s extreme measures to control the illness, she said.”

Their reporting continued: “Past efforts at large-scale quarantines have been largely unsuccessful. Nurses had to tend to the every need of health-care workers who were quarantined in Taiwan during the SARS outbreak, using a tremendous amount of resources, Nuzzo said. A quarantine in Liberia during the Ebola outbreak in 2014 resulted in mass upheaval, and the government quickly pivoted to a milder approach, Nuzzo said.”

Why didn’t the WHO declare it a global health emergency?

After deliberating, the WHO announced Thursday that while there was “an emergency in China,” the outbreak did not yet rise to the level of a global health emergency, given the limited information about the severity of illness and the extent of human-to-human spread.

“The fact that I’m not declaring a [public health emergency] today should not be taken as a sign that WHO does not think the outbreak is serious or that we’re not taking it seriously,” WHO Director General Tedros Adhanom Ghebreyesus said Thursday. “Nothing could be further from the truth. WHO is following this outbreak every minute of every day at country, regional and global levels. We’re working to prevent human-to-human transmission.”

Tedros reported that one-fourth of the infected went on to develop severe symptoms, while the majority of those who died had underlying health problems, such as hypertension or diabetes, which can weaken immune systems.

WHO officials said they could reconvene to make a decision based on additional information.

What’s happening at airports?

Chinese authorities are screening people at airports for coronavirus symptoms. Other airports in Asia are doing the same. North Korea has entirely banned foreign tourists, the majority of whom are Chinese nationals and travel via China, as a precaution.

Federal health authorities in the United States announced Friday that they would immediately begin screening passengers for the virus who are flying into three international airports popular with Chinese travelers — Los Angeles, San Francisco, and New York’s John F. Kennedy. On Tuesday, they added international airports in Chicago and Atlanta to the list.

The screening includes taking temperatures; those with high temperatures could be singled out for additional tests. While screening for a common virus usually takes just hours, health authorities told The Post’s Lena H. Sun that people with suspected cases could miss their connecting flights as the testing could take up to a day.

What about face masks?

Masks intended to filter out airborne particles, like surgical masks, are useful but have a limited effect, according to public health officials. In the United States, where the threat of contracting the virus is low, they are not needed; but in China, health officials are recommending people wear them.

“Wearing a mask walking around isn’t going to do any good, but if you’re in a situation where you’re highly exposed, a mask is helpful,” said Colleen Kraft, associate chief medical officer for Emory University Hospital. “You may wear a mask when someone is going to cough directly on you or [in] a place with a lot of ill people. In a hospital, we wear a mask with patients who have influenza.”

Infectious disease experts stressed that masks need to be properly put on and off in order to be effective.

In Asia, wearing face masks is socially acceptable and common when people don’t feel well, he said. Chinese authorities have urged everyone to wear them. But their effectiveness also depends on the pathogen. For tuberculosis and measles, which are spread through very tiny droplets, face masks may not be helpful because the droplets can reach through the gauze or slip in around the mask.

“There is this concern, though,” said Tom Frieden, former director of the Centers for Disease Control and Prevention. “If someone coughs on you, and you get a gob of virus on your mask, and then you take off your mask, put it on your finger, and then you touch your nose, you could get infected.”

The Centers for Disease Control and Prevention (CDC) recommends that any health-care workers interacting with coronavirus patients or suspected cases wear a stronger kind of mask, known as the N95 respirator, along with other gear such as gloves and eye protectors.

What happened with SARS and MERS?

In November 2002, the SARS epidemic began spreading through China: Over eight months, it moved to more than two dozen countries, killed 774 people, and infected more than 8,000 people. Health authorities say that the “Patient Zero” came in contact with an animal in China’s Guangdong province, which borders Hong Kong.

The virus was finally contained in summer 2003. Health-care workers made up about 20 percent of victims in areas heavily hit with the disease, according to the WHO. There’s still no cure for the disease, but the initial outbreak was contained by isolating suspected patients and screening passengers traveling from infected areas or those suspected of having symptoms.

One factor hindering initial efforts to contain the virus were the limits on coverage of the epidemic implemented by Chinese authorities.