HONG KONG (BLOOMBERG, NYTIMES) - More than 3,000 travellers on two coronavirus-stricken Carnival Corp cruise ships are returning home, fanning out to more than 40 countries and fuelling fears of further contagion from the deadly virus.

The United States on Monday (Feb 17) began evacuating its citizens from the Diamond Princess off Yokohama, Japan, where 70 new cases of the virus were confirmed over the weekend. Canada, Hong Kong and other countries are also planning similar evacuations of hundreds.

Last Saturday, an 83-year-old American woman tested positive in Malaysia, a day after she and more than 2,200 others were cleared to leave the Westerdam cruise ship in Cambodia. The ship arrived there after being turned away by five other ports.

"This woman was on the boat and was infected for a few days - she could have potentially exposed other people on the boat who have now gone home," said Dr Stanley Deresinski, a Stanford University professor and infectious disease specialist at the university hospital.

"There's a possibility that anyone who is infected and asymptomatic could start a chain of infection wherever they return to," he added.

The startling number of cases on the Diamond Princess, which accounts for the biggest cluster of coronavirus infections outside of China, and the newly detected case from the Westerdam raise questions about the effectiveness of containing the virus on cruise ships.

Those concerns have prompted a growing number of Asian countries to block the luxury liners from their ports, threatening earnings of companies like Carnival and Royal Caribbean Cruises.

"Cruise ships are very high-risk for transmission of illnesses," said Prof Jean-Paul Rodrigue, a professor of transport geography at Hofstra University in New York. "People are circulating all over, you're using the same corridors, touching the same handles and railings. It's easy to catch something."

The Westerdam spent two weeks in limbo before Cambodia permitted the ship to berth last Thursday. Passengers were allowed to leave the next day without a quarantine, and the American woman then travelled to Kuala Lumpur to seek a flight back to the US.

All but the American woman and her husband were eventually allowed to continue to their destinations, including airports in the US, the Netherlands and Australia.

Malaysia has since said it won't allow any more travellers from the Westerdam to transit through the country, causing the cancellation of three charter flights to shuttle passengers out of Cambodia.

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Health experts are raising alarm about the Westerdam passengers after their release without a quarantine order at a time when the death toll from the disease has topped 1,700 globally. Most countries planning evacuations of those on the Diamond Princess in Japan have also announced plans for quarantines upon return.

Westerdam passengers should at minimum be monitored by local health authorities and quarantine themselves, Prof Deresinski and other health experts said.

The authorities could also consider testing returning travellers from the ship if feasible, he added. "It doesn't require prolonged exposure to be infected."

The incubation guideline for coronavirus is two weeks - nearly the entire time the ship was at sea after departing Hong Kong on Feb 1 and becoming a floating pariah.

Because the ship was at sea for the required 14-day quarantine, passengers are free to travel after they leave the ship, a US State Department spokesman said. Holland America Line - owned by Miami-based cruising giant Carnival - has maintained there was no known case of the disease during the voyage.

Knowing when the American woman was infected and by whom is key to determine the risk that other passengers may come down with the virus.

"I don't know enough about this patient to know if it is possible they were infected prior to boarding, and had a long incubation period," said Prof Ben Cowling, a professor of epidemiology at Hong Kong University.

"Perhaps it is more likely that they were infected on board, which suggests that there was at least one other case on board i.e. the person who infected them."

The US doesn't have "sufficient evidence" to determine when she may have been exposed or where, said the State Department spokesman.

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CRUISE SHIP OUTBREAKS

The infected woman was cleared to travel from Cambodia after filling out a health questionnaire and undergoing temperature checks.

After she and 144 other Westerdam passengers arrived in Malaysia to transit to other flights, the woman had a fever, was coughing and had difficulty breathing. Two tests confirmed the diagnosis, according to the country's health ministry.

Her 85-year-old husband tested negative, as did six other passengers who were quarantined.

A strict quarantine on the Diamond Princess, operated by Princess Cruises also owned by Carnival, hasn't mitigated infections, which have surged on the ship, while remaining contained in Japan.

US passengers leaving the ship will be subject to an additional 14-day quarantine. Those displaying symptoms will remain in Japan for quarantine, while anyone not boarding the flights will have to wait until March 4 at the earliest to return.

More than 600 Americans on the Westerdam are making their way home, the biggest group by nationality. About 233 guests and 747 crew remain on the ship in Sihanoukville, Cambodia, out of 2,257 who were on the voyage, according to a Holland America Line statement.

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Mr V. Padmanabha Rao, 81, and his 20-year-old grandson are still waiting for details on when they can leave the ship. The captain announced on Sunday afternoon that remaining passengers will likely face a delay, as Cambodian authorities are planning additional health screenings that may include throat swabs, Mr Rao said.

He and the other remaining passengers have been discussing the evacuations from the Diamond Princess, and hope their governments will do the same for those still in Cambodia.

"You can't believe how crazy it's been," said Mr Rao, who lives in Fredericksburg, Virginia. "The uncertainty is the worse part of everything."

The retired thoracic surgeon said he and his grandson plan to quarantine themselves voluntarily for 14 days after getting home.