UPDATE: WILLEMSTAD, Curacao — Authorities in Curacao on Saturday boarded a ship that arrived under quarantine to start vaccinating people to prevent a measles outbreak.

Health officials said only those who already have been vaccinated or have previously had measles will be free to leave the 440-foot ship Freewinds, which belongs to the Church of Scientology.

Curacao epidemiologist Dr. Izzy Gerstenbluth told The Associated Press that a small team is assessing more than 300 people aboard the ship, and that the process might take more than a day.

"We will go on board and do our job," he said, adding that authorities have an international obligation to avoid spreading the disease. "If we allow that to happen, measles spreads in places where the risk of severe complications is much bigger, especially when we're talking about poor countries where people have a lower level of resistance."

Authorities worry people aboard the ship might have been exposed after a female crew member was diagnosed with measles after coming back from Europe. Gerstenbluth said she arrived on the Dutch Caribbean island of Curacao on April 17 and visited a doctor April 22 for cold symptoms. A blood sample was taken and sent to nearby Aruba, where officials confirmed it was measles on April 29, a day after the ship had departed for St. Lucia. Curacao health officials then alerted authorities in St. Lucia.

The Freewinds was under quarantine in St. Lucia earlier this week before it returned to its home port of Curacao early Saturday.

Gerstenbluth said it would be easy to spread the disease given that it's a small ship.

"This is what happens when we don't vaccinate," he said.

Symptoms include runny nose, fever and a red-spotted rash. Most people recover, but measles can lead to pneumonia, brain swelling and even death in some cases.

Measles has sickened more than 700 people in 22 U.S. states this year, with federal officials saying the resurgence is driven by misinformation about vaccines.

Church officials have not returned calls for comment. According to the church's website, the ship is the home of "a religious retreat ministering the most advanced level of spiritual counseling." It says religious conventions and seminars also are held aboard.

PREVIOUS STORY: A cruise ship with nearly 300 passengers and crew was ordered quarantined in the Caribbean port of St. Lucia after a case of measles was confirmed on board, island health officials said Wednesday.

One female crew member has a confirmed case and St. Lucian authorities said they've been working in close consultation with the Pan American Health Organization (PAHO) and Caribbean Public Health Agency (CARPHA).

The ship has been under quarantine since Monday morning, officials said.

"Measles in a highly contagious disease. Anyone who is not adequately immunized against measles can contract the disease, if there has been close contact with a confirmed case," according to a statement issued Wednesday by Dr. Merlene Fredericks-James, the island nation's chief medical officer. "It is therefore likely that other persons on the boat may have been exposed."

"The ship’s doctor has the confirmed case in isolation on the ship," Dr. Fredericks-James added. "The individual is in stable condition."

St. Lucian health officials declined to name the ship involved.

But St. Lucia Coast Guard Sgt. Victor Theodore told NBC News that the vessel involved is named "Freewinds," which is the name of a 440-foot cruise ship owned and operated by the Church of Scientology

Theodore identified the quarantined craft as the same one that's listed on the church's website.

While no one has been allowed to leave the ship, St. Lucian authorities said they don't have the authority to keep it from leaving.

For now, the ship is scheduled to leave St. Lucia at 11:59 p.m. on Thursday, officials said.

"The ship can leave our port, but we have no jurisdiction over their next destination," Dr. Fredericks-James told NBC News.

A Scientology spokesperson could not be immediately reached for comment on Wednesday.

The Pan American Health Organization said it's aware of the measles case on this cruise ship, but declined to comment or reveal any additional details.

The once-vanquished disease has made a surprising comeback in recent months, with more than 700 cases reported in the United States this year, officials said.

It's the highest number of reported cases in the U.S. in a year since 1994. The had been declared eliminated in 2000.

Southern California health officials have even asked fans of the blockbuster movie “Avengers: Endgame” to check their ticket stubs.

That's because a California woman in her 20s, with a confirmed case of measles, went to the see the superhero movie at the AMC in Fullerton. Anyone who was at that theater between 11 p.m. Thursday to 4 a.m. on Friday is being urged to get a screening.