Japan already had several confirmed coronavirus cases when a giant cruise ship arrived at the port of Yokohama last week.
Now, with the disclosure that 61 people from that ship have tested positive for the virus, Japan is scrambling to prevent a larger outbreak even as it is also preparing to welcome hundreds of thousands of visitors for the Summer Olympics starting in Tokyo in July.
The Japanese government on Monday quarantined the ship, the Diamond Princess, with more than 3,700 crew and passengers aboard, after learning that a man who had disembarked in Hong Kong on Jan. 25 had tested positive for the virus.
In the days since, it has become a fixture in Yokohama’s harbor, making a slow circuit toward shore for supplies and then back out to sea for quarantine.
Japanese health workers screened 273 people from the ship who showed symptoms or who had come into contact with the infected man. The health ministry said on Friday that 61 people were infected with coronavirus and had been taken to hospitals across several prefectures.
The remaining passengers and crew members have not yet been tested and will remain quarantined for 12 more days. Some passengers shared concern about the virus’s spread and about the days ahead stuck in their cabins.
Masako Ishida, 61, told The New York Times on Friday that all passengers had been given a thermometer and instructed to report any reading over 37.5 degrees Celsius (99.5 Fahrenheit).
“We were screened on the first day by quarantine officers, but there hasn’t been anything ever since,” she said. “We’re told to report immediately when we feel we have a fever.”
Ms. Ishida, who is traveling with her husband and mother, said she was trying to remain calm. But she was also looking forward to leaving her cabin.
“Nobody gets out of their room unless they’re in a selected group who get to go out to the deck to breathe fresh air at designated times,” she said, “So I’m hoping that the infection will stop.”
Gay Courter, 75, an American, said that she was “worried and upset” by the growing number of coronavirus cases onboard the ship.
“There’s been some terrific memories, but now it’s turned into something catastrophic,” she said of the trip, which she made with her husband, Philip Courter.
“I would be happy if they come around and do tests,” she said, referring to the quarantine officers. “They only took our temperature once three days ago, and asked us to fill out a form. They did not swab anybody except for people who had temperatures.”
She emphasized that the crew “are doing fabulous jobs, considering they’re at risk too.”
She also noted the potential consequences of a wider outbreak in Japan.
“If people on the ship start dying, nobody’s going to come to the Olympics,” she said.
Cruise ships regularly make the news as places where infections spread rapidly. But the high rate of coronavirus cases among those tested onboard the Diamond Princess concerned infectious disease specialists.
“That’s worrisome,” said Dr. Peter Rabinowitz, who is co-director of the University of Washington MetaCenter for Pandemic Preparedness and Global Health Security. “That’s a lot of positive cases.”
Dr. Rabinowitz said that given the relatively low rate of testing among all passengers, it was possible that other cases had been missed. “I would think that this would be a very serious situation for the public health department to be evaluating and doing all they can to try to limit the spread,” he said.
The Japanese health minister, Katsunobu Kato, said that officials were consulting with infectious disease experts about whether to screen additional passengers on the Diamond Princess. The focus would be on the elderly, those with underlying diseases and those who had come into close contact with the 61 infected passengers.
The main question for Japan was how to keep the virus from spreading more broadly within the country just months before the Olympics.
Masaya Yamato, an infectious disease specialist at Rinku General Medical Center in Osaka, said he was not yet concerned about a widespread outbreak in Japan. He noted that one hospital that had treated three coronavirus patients had said that all of the cases were quite mild.
Public health specialists said that it would be difficult to draw too many conclusions from what appeared to be a high rate of infection on the cruise ship.
“The high ratio might suggest they were very good at first assuming who was most exposed,” said Karen Eggleston, director of the Asia health policy program at the Shorenstein Asia-Pacific Research Center at Stanford University. “On the other hand, if there were others that were equally exposed that were not tested, that would mean it was very troubling.”
The Japanese government has asked another cruise ship, the Westerdam, not to make a scheduled stop in Okinawa. When the Diamond Princess stopped in Naha, Okinawa, last Saturday, about 2,600 passengers left the ship for a few hours and traveled by buses and taxis. Health officials are trying to trace their routes.
Efforts to track people with potential exposure to the virus is one of the main public health measures used to contain the spread of the illness.
Dr. Yamato said that public health officials had so far been able to trace the activities of people who had tested positive for the virus in Japan. “But we might eventually see patients that we cannot trace back,” he said. “Some people might not get tested, or some who don’t have severe symptoms might be diagnosed as having an ordinary cold.”
Japan has also recorded 25 cases of coronavirus infections among Japanese citizens who had returned from Wuhan on charter planes over the past 10 days or who had come into contact with tourists from Wuhan. A fourth charter flight that returned to Tokyo on Friday had 198 passengers aboard, including non-Japanese spouses and family members.
Four passengers were taken to hospitals after landing in “poor physical condition.” The others are undergoing health checks at the Center Hospital of the National Center for Global Health and Medicine.
Prime Minister Shinzo Abe told Parliament that the government would “compile emergency countermeasures as early as next week,” but he did not give details.
Experts said Japan’s health care system should be equipped to handle the number of cases in the country right now.
About 60 facilities across Japan have test kits for the coronavirus, and the hospitals that have accepted patients have specialized isolation units and staff members trained to handle infectious diseases.
So far, among people who have returned from Wuhan, the government is asking those who do not show symptoms or test positive for the coronavirus to isolate themselves voluntarily for 14 days after returning to Japan.
But at least two people who returned on the first charter flight declined to take a test.
Determining whether to mandate quarantines or simply request voluntary isolation can be a difficult decision for officials.
“Someone is going to have to weigh both the public health consequences of requiring hundreds of people to be in quarantine and the political consequences,” said Timothy Brewer, professor at the schools of medicine and public health at University of California Los Angeles. “Most people who end up in quarantine probably have not been exposed and do not have the disease — so you’re restricting the civil liberties of a lot of people.”
But Mr. Brewer said that the mortality rate based on available data — roughly estimated around 2 percent — could warrant a more vigorous approach.
For now, experts said there was no need to close schools, cancel all concerts and sports events, or order workers to stay home. But such measures should be part of an evolving decision, Dr. Eggleston said.
“It’s the responsibility of individuals to consider not only their own well-being but those of others around them,” she added. “Share information, report symptoms, submit to testing and quarantine — and empathize and support those who are doing so on behalf of the community.”
Hisako Ueno and Isabella Kwai contributed reporting