The arrival of the Westerdam cruise ship in Cambodia seemed like cause for celebration.

After days of circling in the sea, hundreds of elated passengers disembarked in the Cambodian city of Sihanoukville on Feb. 14, amid assurances the ship was free of the coronavirus. President Trump broadcast the news on Twitter, thanking the Cambodian government for its hospitality and tagging Micky Arison, the chairman of the vast multinational cruise company, Carnival Corporation, that owns the Westerdam.

But the joy was short-lived. Over the weekend, an American passenger who had been on the Westerdam tested positive for the virus, raising concerns that infected passengers were dispersing across Asia.

Now the situation is fast becoming a crisis for Carnival. Thousands of miles away, another Carnival ship, the Diamond Princess, has been moored outside the Japanese port city of Yokohama with hundreds of coronavirus cases reported on board. Passengers who tested negative began leaving the ship on Wednesday. American officials had already evacuated some passengers.

“We have protocols, standards and practices for every possible issue you might imagine, including coronavirus,” said Roger Frizzell, a spokesman for Carnival. “But this virus is so new and unknown that everyone, including the medical profession, is learning on the job.”