A dream trip has turned into a floating nightmare for passengers and crew aboard the MV Hondius, where a hantavirus outbreak has killed three and sickened at least four others.

Nearly 150 people still on board are in quarantine after health officials confirmed that two passengers were infected with the Andes strain, a rare and dangerous version of the virus that can spread from person to person.

But Dr. Jill Roberts, a molecular epidemiology expert with the University of South Florida College of Public Health in Tampa, told The Post that there’s another virus she considers an even greater threat if it ever reached a cruise ship.

“The one that keeps me up at night is measles,” she said. “It would be unlikely that any unvaccinated person on a cruise ship would not catch the virus if present.”

Ironically, the idea of a “wellness cruise” — one on which many people are unvaccinated — is one of the potential “scenarios I hate most” when it comes to health, she said, because measles could infect a lot of people easily.

Considered one of the most contagious diseases known to medicine, measles spreads through the air when an infected person coughs, sneezes or even talks, releasing droplets that others can inhale. It can also spread by touching contaminated surfaces and then touching the eyes, nose or mouth.

Adding to the risk, the virus, which attacks the respiratory system, can remain viable in the air or on surfaces for up to two hours after an infected person has left the area.

That combination makes cruise ships especially vulnerable, with experts describing them as an “ideal” environment for infectious disease.

“Microbes can easily travel person-to-person in the crowded conditions of a cruise ship,” Roberts said. “People underestimate the severity of the measles virus.”

Fortunately, though, the disease is vaccine preventable.

“The vaccines are not 100% effective, but they are close at 97%, making disease highly unlikely in vaccinated individuals,” she said.

It spreads before you know it

For those who are not vaccinated, Roberts warned that the danger escalates fast: “By the time you realize measles is on the cruise, it will be too late for prevention.”

Infected people can spread measles during an incubation period when they feel completely fine, with symptoms typically appearing seven to 14 days later.

If exposed, about 9 in 10 unvaccinated people will become infected, often developing fever, cough, runny nose, red watery eyes and a blotchy rash.

Complications can be severe, including pneumonia and swelling of the brain, with the highest risk among young children, older adults, pregnant women and immunocompromised people.

Measles love cruise ships

Think a cruise ship outbreak seems unlikely? There have already been several documented cases. In 2014, at least 34 people became ill with measles aboard the Costa Pacifica in the western Mediterranean, leading to additional cases on land after passengers disembarked.

And in 2019, doctors aboard the Freewinds, a vessel owned by the Church of Scientology, administered 100 doses of the measles vaccine to people on board after a staff member tested positive for the highly contagious disease, forcing more than 300 people into quarantine.

If measles were ever detected at sea, Roberts said unvaccinated passengers would likely need to isolate for 21 days under close monitoring to watch for symptoms.

Treating anyone infected on board would present challenges of its own. Vaccines and immunoglobulin treatments can be used, but Roberts points out that neither is likely available on a ship.

“Likewise, measles can be treated medically but treatments are not likely available on a cruise ship,” she said.

That limitation, she added, could lead to worse outcomes for patients.

Roberts’ comments come as measles vaccination rates in the US have slipped below the 95% threshold needed for herd immunity.

CDC data shows kindergarten vaccination coverage has fallen from 95.2% in 2019 to 2020 to 92.7% in 2023 to 2024.

At the same time, measles cases are climbing. The CDC has confirmed 1,814 cases across 36 states as of April 30, with 24 new outbreaks reported so far in 2026.

That already represents 79% of last year’s total in just five months, with 2025 marking the highest measles levels nationwide since 1991.