It is too early to tell whether the decision to let hundreds of passengers from the Westerdam fly off has the makings of an epidemiological disaster. Cambodian health authorities said that 409 of the 2,257 passengers and crew had left Cambodia for their homes scattered across the globe. The rest remain in hotels in Phnom Penh, the capital, or on the ship.

But deficiencies in screening for the coronavirus aboard the ship, along with continued complacency about the epidemic in Cambodia, are raising fears this small Southeast Asian nation could prove to be a surprising vector of transmission for a virus that has already killed more than 1,700 people, mostly in China, the epicenter of the outbreak.

Many health experts urge people who have been in contact with coronavirus patients to self-quarantine for 14 days, lest they add another spoke to the contagion network.

But on Monday, Mr. Hun Sen directed officials in Phnom Penh to treat passengers from the Westerdam to a sightseeing jaunt.

“To tour the city is better than staying in rooms or at the hotel feeling bored or scared,” said a post on Mr. Hun Sen’s Facebook page.

The lack of urgency in Cambodia, where officials milled around the ship on Monday without protection, points to the obstacles in trying to contain a virus that experts warn is spreading faster than SARS or MERS.

“This is influenza-like transmission,” said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. “It’s like trying to stop the wind.”