An American woman who left a cruise ship in Cambodia last week and flew to Malaysia with more than a hundred other passengers has tested positive for the coronavirus, alarming health experts who fear that some exposed passengers who then traveled onward could become a new source for global transmission.

In all, 1,455 passengers and 802 crew members were aboard the Westerdam, a Holland America Line cruise ship that left Hong Kong on Feb. 1. It was at sea for just under 14 days, the time frame that is believed to be the maximum incubation period for the highly transmissible virus.

Cambodia allowed the ship to dock after it was repeatedly turned away by other countries over coronavirus concerns. Officials said that more than 140 other passengers from the Westerdam had flown by Saturday from Cambodia to Kuala Lumpur, Malaysia’s capital. All but the American woman and her husband were eventually allowed to continue to their destinations, including airports in the United States, the Netherlands and Australia.

As of today, 233 passengers and 747 crew members were still on the ship docked at Sihanoukville, Cambodia, Holland America said. The more than 1,000 other passengers departed Sihanoukville on charter flights to Phnom Penh, Cambodia, and were in various stages of transit home, the cruise line said.

It was unclear whether Cambodia would seek to quarantine passengers who are still in the country, or whether those who had left by plane would face quarantine in their own countries when they arrived.

“We anticipated glitches, but I have to tell you I didn’t anticipate one of this magnitude,” said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University Medical Center, adding that the best approach to containing a broader spread of the virus from the Westerdam would be to track down all of the passengers and quarantine them for two weeks.

“This could be a turning point” in the epidemic, Schaffner said. “We have potentially many people in many countries, and all it would require is just the establishment of another outbreak in another country, and that could potentially tip the scales.”

Dr. Eyal Leshem, director of the Center for Travel Medicine and Tropical Diseases at the Sheba Medical Center in Israel, called the disclosures “extremely concerning” and said the passengers who traveled onward from Kuala Lumpur substantially increased the risk of a pandemic.

“We may end up with three or four countries with sustained transmission of the virus,” he said. “It may be more and more difficult to make sure this outbreak is contained only within China.”

Cambodia’s prime minister, Hun Sen — who has argued that fears about the coronavirus are overblown and refused to evacuate Cambodian students from Wuhan, the Chinese city where it emerged — personally greeted many Westerdam passengers with flowers when they disembarked on Thursday. But he may have put his own citizens at risk as a result.

The biggest cluster of cases so far outside China, where the virus emerged, is on another cruise ship, now off Yokohama, Japan. And hundreds of passengers from the ship docked in Cambodia, the Westerdam, have already departed for several countries.

Malaysia’s deputy prime minister, Wan Azizah Wan Ismail, said at a news conference on Sunday that the American woman, who is 83, had tested positive twice for the virus.

The woman and her husband, 85, also an American citizen, were both hospitalized and in isolation. The husband has also been tested twice for the virus, and the results were negative both times. But he has pneumonia, which is often a sign of the virus that appears before it can be identified through testing.

The Westerdam was allowed to dock in Kaohsiung, Taiwan, on Feb. 4, but was turned away in Taipei, Taiwan, and by ports in Japan, the Philippines, Thailand and the U.S. territory of Guam, according to local news media reports and a relative of passengers onboard. With Holland America insisting that no one on the Westerdam was infected, Cambodia agreed on Thursday to let it dock.

After Malaysia announced that the American passenger had become ill, the Kaohsiung Department of Health asked taxi drivers who had driven passengers to report in for health checks, The Taipei Times reported.

The global fight against the coronavirus is complicated by the fact that different countries may have different levels of disease surveillance and prevention measures. While the World Health Organization provides guidance, it is up to each country to enforce these standards, including whether to quarantine people who may have been exposed or to stop them from traveling.

The Cambodian government said passengers and crew members were screened using protocols of the World Health Organization and the Centers for Disease Control and Prevention in the United States before being allowed to disembark.

Their temperatures were checked, and about 20 people who had reported being sick during the trip were tested for the coronavirus, according to a statement issued Sunday by the U.S. Embassy in Cambodia. All of them tested negative.

Schaffner said it might have been wiser to test each disembarking passenger, because other screening methods — like questionnaires and temperature taking — are fallible. People eager to get off a ship may not respond entirely truthfully to questioning, and sick people’s temperatures vary during the day, he said, generally being lower in the morning.

The woman who tested positive for the virus did not visit the ship’s medical center during the cruise to report any symptoms of illness, Holland America said in a statement on Sunday.

The woman and her husband were among 145 passengers from the ship who then flew to Malaysia. All went through thermal scanning at the Kuala Lumpur airport, and 137 were allowed to fly on to other destinations, officials said.

But the American couple and six other passengers were stopped and tested for the virus. The other six were later cleared for travel.

After the American woman initially tested positive, both Holland America and Cambodia questioned the result, calling for further testing and confirmation. Malaysia carried out a second round of testing, which officials said on Sunday had confirmed that the woman was infected.

Malaysia’s deputy prime minister said that the country would not accept any more passengers from the Westerdam, which is still docked in Sihanoukville, Cambodia.

“If there’s one passenger who is confirmed, the others are potentially in trouble,” said David Hui, director of the emerging infections disease center at the Chinese University of Hong Kong. He recommended that the other passengers be quarantined in their home countries for 14 days.

The World Health Organization is working with local authorities to have samples sent to regional reference laboratories for further testing said a spokesman, Tarik Jasarevic.

President Donald Trump tweeted his thanks to Cambodia for allowing the ship, more than 600 of whose passengers were Americans, to dock.

Many of the passengers went sightseeing in Sihanoukville after the ship docked in Cambodia, visiting beaches and restaurants and getting massages. On Sunday, after Malaysia’s announcement, the remaining passengers and crew members were restricted to the ship, and buses that had been scheduled to transport them were parked nearby.

Coordination between Malaysia and Cambodia appears to have been minimal. In a letter to his Malaysian counterpart on Sunday, the health minister of Cambodia, Mam Bunheng, said he had learned through the news media that the first test of the American woman had been positive.

Attempts to contact Cambodian officials for comment were not immediately successful.

Holland America said in its statement today that no other passengers or crew members had reported any symptoms and that passengers who had returned home would be contacted by their local health departments. There were no details on how that would be arranged.

“We are in close coordination with some of the leading health experts from around the world,” said Dr. Grant Tarling, chief medical officer for Holland America Line. “These experts are working with the appropriate national health authorities to investigate and follow up with individuals who may have come in contact with the guest.”

The company said that before the passengers’ departure, the passports of everyone on board had been reviewed to make sure that no one had traveled through mainland China in the 14 days before the cruise.

The company defended the health screening it had conducted during the cruise and on arrival in Cambodia. But it did not respond to a question on whether it had been appropriate to let Westerdam passengers travel to many parts of the world without putting them in quarantine first.

Christina Kerby, 41, a communications director with BlueShield in California, said she was among a group of passengers who had nasal and throat swabs taken in Phnom Penh today. Kerby was supposed to fly to Singapore today and then on to San Francisco.

“The stress has absolutely taken its toll,” Kerby said by telephone. “I certainly don’t feel like myself after the roller coaster that we’ve been through.”

Kerby said that her temperature had been taken two or three times during her stay on the ship, and that passengers were required to fill out health questionnaires detailing whether they had symptoms such as cough, fever and diarrhea.

“I can’t really comment on how this was missed, but I did feel very safe and well cared for on the ship,” she said, adding that she believed Holland America “was operating appropriately given the situation.”

Kerby said she had discussed the risk of going on the cruise with her family. She boarded the ship in Hong Kong and traveled with her 75-year-old mother and her brother.

“We made the decision that it’s not worth passing up the potential to have a lot of fun and see the world just out of fear,” she said. “That’s why I joined, and I think the other passengers have the same feeling.”

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