Editor’s note: A CDC official confirmed to TIME a week after this story was originally published that the U.S. passenger did not have COVID-19 and called earlier results a false-positive.

The elderly cruise passenger who tested positive for coronavirus after disembarking from a U.S. cruise ship in Cambodia has raised the specter that other passengers now dispersed around the world could be infected too.

Currently, hundreds of passengers from Holland America Line’s Westerdam are being held back in Cambodia, where the ship was allowed to dock last week after being denied entry to multiple ports following rumors of a possible coronavirus on board—despite assurances from the cruise line that there were no signs of the virus, officially named COVID-19, in any passengers.

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After the shipped docked in Sihanoukville, Cambodia, on Feb. 13, twenty lab tests for the virus that had been conducted on board came back negative and passengers were cleared to begin disembarking after they were screened for symptoms of the virus. They were greeted with handshakes by Cambodian Prime Minister Hun Sen, who invited them to tour Cambodia. Some left for a hotel in the capital, Phnom Penh, while others remained on the ship. Hundreds more boarded flights for destinations around the world—either returning home or moving on to other vacation destinations.

Then, on Sunday came the news that an 83-year-old American passenger on the ship tested positive for coronavirus in Malaysia after she was stopped while going through thermal temperature scanners at Kuala Lumpur airport.

Her 85-year-old husband, who is also an American citizen, has been tested twice for the virus, but the results were negative.

Officials in Cambodia halted all further passengers from the Westerdam from leaving the country. Passengers still aboard the ship have been prevented from disembarking and authorities have asked those in Phnom Penh to remain in their hotel rooms.

Despite those measures, hundreds of passengers who may have come into contact with the infected woman on the ship are already scattered across the world.

A 66-year-old passenger from Australia, Ann-Maree Melling, tells TIME that after leaving the Westerdam, she and her husband stayed in Phnom Penh in Cambodia before flying to Bali, where they are now vacationing. The couple’s flight had a stopover in Singapore.

Melling said that she and her husband plan to check in with their doctor when they get home later this month. “We both feel it would be irresponsible not to,” she says.

U.S. State Department officials said that 200 Americans remain in Cambodia, waiting to be cleared for travel, including 92 who remain on board the Holland America Line ship the Westerdam. Cambodian officials asked those in hotels in the country not to leave their rooms while further testing is done.

According to the company, all passengers and crew were screened for illnesses and had their temperatures taken multiple times while on board, and upon disembarkation the passengers underwent an additional health screening and were required to fill in a written health questionnaire.

“We are in close coordination with some of the leading health experts from around the world,” Dr. Grant Tarling, Chief Medical Officer for Holland America Line, said in a post on its blog. “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.”

On Monday, Holland America Line said Cambodian Health officials have begun testing the 255 guests and 747 crew members that are still on the ship. The cruise line said it expects that it will be several days before the remaining passengers are allowed to leave the boat.

“At this time, no other guests or crew on board or at the hotel have reported any symptoms of the illness,” Holland America Line said.

Christina Kerby, from Alameda, Calif. was supposed to be home on Feb. 16, but she is among those stuck in a hotel in Cambodia. Some other passengers have been cleared to travel, but Kerby hasn’t yet been told when she’ll be able to head home.

“I’m preparing for the worst, that if something does happen we might be kept here longer,” she tells TIME.

But Kerby, 41, says that she understands that need for caution.

“I’m willing to comply with whatever is needed to protect the health and safety of the people here and back home,” she says. “At the same time, I really miss my kids and my husband and I want to get home.”

It is unclear when or where the infected woman caught the virus, which has sickened more than 73,000 and killed 1,873 people. On Tuesday, China reported about 1,900 new cases of the virus and 98 additional deaths. The cruise line issued a statement saying that the woman did not appear sick while on board.

“During the voyage there was no indication of COVID-19 on the ship,” the cruise line said on its blog. “The guest who tested positive did not visit the ship’s medical center to report any symptoms of illness.”

The Cambodian government said everyone on the ship had been screened for the virus in collaboration with the World Health Organization and the Centers for Disease Control and Prevention. Melling said she was given a health questionnaire and had her temperature taken before disembarking in Cambodia, in Singapore and again upon arrival in Bali.

But COVID-19’s sometimes mild symptoms mean that it can be difficult to detect. And estimates vary for the average incubation time of the disease from around 3 to 5 days, though some researchers in China say that in rare cases, the incubation period could be as long as 24 days.

Previously, Japan, the Philippines, Taiwan, Thailand, and the U.S. territory of Guam had all denied the ship’s entry on concerns over the outbreak, despite repeated statements from cruise officials that there were no known cases of the virus on board. Cambodia eventually agreed to allow the ship to dock.

The Westerdam is not the only cruise ship to be caught in the middle of the coronavirus outbreak. The virus-stricken ship Diamond Princess was put under quarantine at the Yokohama Bay in Japan on Feb. 5 and infections aboard continue to rise. As of Sunday, at least 355 of the 3,700 passengers and crew are confirmed infected with COVID-19. At least 40 Americans are among those infected, according to Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases at the National Institutes of Health.

Reuters reported on Friday that Vietnam turned away two cruise ships — one of which was the German ship AIDAvita — out of concerns about potentially spreading the coronavirus. One of the ships headed to Thailand, per Reuters.

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Write to Amy Gunia at amy.gunia@time.com, Hillary Leung at hillary.leung@time.com and Madeleine Carlisle at madeleine.carlisle@time.com.