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China has confirmed more than 4,500 cases of the virus, with more than 100 deaths, as of Tuesday, January 28. Most have been in the central city of Wuhan, where the outbreak began in December.

In the United States, five confirmed cases (though no deaths) have been reported. New Hampshire officials said two people are being tested for suspected cases. No cases have been confirmed in Massachusetts, and passengers arriving at Logan Airport from China are being screened.

Here’s what you need to know.

What’s coronavirus?

“Coronaviruses are a large family of viruses that mostly infect animals,” said Dr. Daniel R. Kuritzkes, chief of the Division of Infectious Diseases at Brigham and Women’s Hospital.

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The varieties that have infected humans usually lead to minor ailments, such as the common cold or gastrointestinal distress. But in recent years, strains of coronavirus that jumped from animals to people have caused serious illness and death.

Two high profile outbreaks in recent years were caused by coronaviruses.

In 2002, the outbreak of severe acute respiratory syndrome, or SARS, led to more than 8,000 respiratory illnesses and 774 deaths. And since 2012, MERS — Middle East respiratory syndrome — has sickened 3,000 people and caused 858 deaths, mostly in Saudi Arabia. The SARS outbreak was eventually contained, but MERS continues to cause illness.

Then, on Dec. 31, a Chinese provincial health department notified WHO of a cluster of unusual pneumonia cases, later identified as a new coronavirus, still unnamed. Most of those affected had visited an animal market, and animals are the likely source.

But concerns were heightened when it became clear that the virus can be transmitted from one person to another.

How serious is this new illness?

In short, it’s too soon to tell.

Dr. Larry Madoff, medical director of the state Department of Public Health’s Bureau of Infectious Disease and Laboratory Sciences, said the new virus concerns him for several reasons: it’s spreading quickly, it can be transmitted person to person, and little is known about it. “There are a lot more questions than answers,” he said. And, he cautioned, because the virus is new, the human immune system has not been primed to fight it.

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So far it seems that only people with very close contact with a sick person, such as family members and medical caregivers, have become infected. However, it’s still not clear how many people have been infected; many may have acquired the virus but had such mild symptoms they never saw a doctor. Most of those who died had other illnesses, or were elderly.

“From the limited information that we have to date it would appear to be less serious than SARS or MERS,” Kuritzkes said.

Another good sign is that it appears likely that the new coronavirus is transmitted, like colds and flu, by droplets from sneezes and coughs, said Dr. Shira I. Doron, an infectious disease physician and hospital epidemiologist at Tufts Medical Center. If the virus traveled through the air, like measles, it would be much more worrisome, she said.

How are health officials preparing, in case the virus comes to Massachusetts?

“We are taking this very seriously. We’re spending lot of time preparing,” Madoff said.

The health department has reminded health-care providers to ask patients about their travels, and hospitals were already routinely doing that. They have procedures for isolating a sick person, and gowns, masks, and gloves to protect caregivers.

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“We’ve learned a lot over the years from the experience with SARS, MERS, and Ebola,” Doron said.

Since the emergence of the new coronavirus, Tufts has been doing some refresher training and checking to make sure that staff is asking about foreign travel, she said.

Hospitals were already alert to the possibility of someone from the Middle East bringing MERS, “and it will be very easy to translate that approach to people who might be showing up with the new coronavirus,” Kuritzkes said.

What should residents do in response?

“I don’t think people need to be worried here in the Boston area,” Kuritzkes said. “People need to be appropriately cautious.” That means if you know someone who’s been to China and appears sick, keep your distance.

And take the other precautions that make sense at this time of year when respiratory illnesses are already rampant: Wash your hands well and often; don’t touch your eyes, nose, or mouth when out in public; stay home when sick; and stay away from sick people.

“At this point, based on the numbers of patients, and where the cases have been isolated, all Americans are at low risk,” Doron said. “I don’t think the average American needs to be thinking about it more than they would about things they’re more likely to come in contact with, like the flu.”

Indeed the flu season is in full swing, and Madoff said it’s looking very severe. The flu can be deadly: On Thursday, the health department announced that a teenager from Worcester County was the first child to die of the flu this season.

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Emily Sweeney of the Globe staff contributed to this report.










Felice J. Freyer can be reached at felice.freyer@globe.com. Follow her on Twitter @felicejfreyer