Health officials said he is recovering well and is checking in regularly by phone with public health nurses. He is not sick enough to require hospitalization, and doctors decided that transferring him to a hospital for isolation would put other patients at risk of infection. The student will stay in isolation until he is “cleared,” but officials don’t know how long that would take or how they will determine that he is no longer contagious.

The student had close contact with a small number of other people and did not participate in any university activities since his return from China, officials said. His contacts are being monitored for symptoms.

Health officials did not provide details of the student’s whereabouts before he was isolated, but noted the disease is not transmitted through casual contact. “This kind of spread requires close face-to-face contact over a period of time,” said Dr. Larry Madoff, medical director of the state health department’s Bureau of Infectious Disease and Laboratory Sciences.

The Boston man is the eighth case of coronavirus reported in the United States; the others include three people in California, two in Illinois, and one each in Washington state and Arizona. In New York City Saturday, officials said they are investigating whether a patient at Bellevue Hospital Center has the virus.

By Saturday afternoon, China reported nearly 12,000 cases and 259 deaths. Outside China, the numbers are much lower, but growing, with nearly two dozen countries reporting cases, most in people who had traveled to China. Of the eight cases in the United States, seven picked up the virus in China. The other patient is the husband of one of those travelers.

Even before the first case in Massachusetts arose, health officials had been girding for the possibility the virus, known as 2019-nCoV, would make its way here.

Hospitals have been updating disaster plans, checking supplies, and retraining staff. The Department of Public Health Friday launched a website for public information, activated an incident command structure to manage the flow of information among agencies, and issued guidance to hospitals, other providers, and local boards of health. Nationally, the Pentagon on Saturday approved the use of US military facilities to accommodate as many as 1,000 people who may have to be quarantined upon arrival from overseas.

Health officials emphasized this new case doesn’t change their belief that the risk to the public in Massachusetts, and the United States, is very low right now.

But as the ground shifts daily with new facts and uncertainties, few are willing to predict the extent of the threat.

“It’s too soon to say whether this will be a real epidemic in Massachusetts, or in the United States,” Madoff said.

Among the unknowns are exactly how the virus is transmitted and how severe it will be.

“It’s challenging to figure out exactly what this is going to ultimately mean for the health care community,” said Dr. Paul Biddinger, director of the Center for Disaster Medicine at Massachusetts General Hospital.

For now, officials take heart in evidence that, outside of China, coronavirus so far has caused only mild illness and has not spread widely. In China, most of those who have died were elderly or already ill.

But a report from Germany on Thursday raised new questions about how coronavirus spreads. Doctors learned that a woman from Shanghai without symptoms had infected a man at a business meeting; she didn’t get sick until she was on her way back to China.

China had reported instances of asymptomatic transmission, but this was the first carefully documented case. And it raised the specter of a virus that could be easily transmitted by people who had no idea they were carrying it.

“This has very important implications for how to control the spread of the outbreak. It may not be possible to contain it if this data pans out,” Biddinger said.

But Madoff said one case doesn’t mean such transmissions occur routinely. “We know people coughing and sneezing and shedding this virus actively are the real drivers of outbreaks,” he said.

Dr. David Hamer, an infectious diseases physician at Boston Medical Center and a professor at Boston University School of Public Health and School of Medicine, said asymptomatic transmission could mean screening efforts will miss some cases.

“You’re still going to catch many of the cases,” Hamer said in an interview before the first Massachusetts case was reported. But, “you’re going to have a net that’s got holes in it.”

Even with some asymptomatic transmission, Hamer believes that Western countries will be able to prevent onward transmission of the virus. He sees a greater risk to countries in Africa, where China has a huge business presence but public health resources are limited.

In the United States, he said, “Most hospitals have mobilized and have rapidly developed plans on how to address these patients.” Boston Medical Center, for example, has been holding regular meetings and produces a daily bulletin about preparedness efforts.

While the virus is new and its course unpredictable, local hospitals have faced similar challenges in the past — from the Boston Marathon bombing to the Ebola scare to the occasional bad flu season.

“For the last five years, we’ve really been training for this possibility,” Biddinger said. In 2015, Mass. General was designated as one of 10 regional treatment centers for Ebola and other special pathogens. Mass. General is sharing its plans and training protocols with other hospitals, he said

Biddinger said supplies of protective equipment that prevent caregivers from getting infected, such as masks and gowns, are adequate, and Mass. General has an emergency cache of equipment for extraordinary situations.

Even as hospitals and health departments ramp up their efforts, a former Massachusetts public health commissioner worries that resources will prove inadequate.

John Auerbach, now chief executive of Trust for America’s Health, a nonprofit concerned with preventing illness and injury, said the United States may find its response hampered by the “fraying” of public health funding for emergency preparedness efforts.

“Those are limiting factors in terms of the ability to really address what might be coming,” he said.

Although the new coronavirus resembles the flu in its chief symptoms — fever, cough, shortness of breath — and the way it spreads through respiratory droplets, there are no antiviral drugs known to treat it, nor is there a vaccine for it. And it seems to move faster through the population than the flu, and possibly with a higher death rate.

These trends have convinced Michael T. Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, that 2019-nCoV cannot be contained.

“We have every reason to expect that what is playing out in China will play out elsewhere,” Osterholm said.

Not everyone agrees. A top official of the World Health Organization told STAT on Saturday that he believed epidemic could still be contained.

And Dr. Robert W. Amler, dean of the School of Health Sciences and Practice at New York Medical College and a former CDC medical officer, advises being “vigilant and concerned, but not alarmed.”

“If you want to protect yourself, we already know how to do that,” Amler said. It’s familiar advice: Stay away from sick people and stay home if you’re sick; cover your coughs and sneezes; and especially, wash your hands often with soap and warm water for at least 20 seconds.

“It’s quite surprising,” Amler said, “how effective handwashing can be.”

John Hilliard of the Globe staff contributed to this report.

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