(CNN) An Israeli flight attendant has slipped into a coma after contracting measles, according to health officials.

The 43-year-old woman has encephalitis , or inflammation of the brain, a well-known and potentially deadly complication of the virus. She was otherwise healthy before getting measles.

"She's been in a deep coma for 10 days, and we're now just hoping for the best," said Dr. Itamar Grotto, associate director general of Israel's Ministry of Health.

The flight attendant, who works for El Al, the Israeli national airline, might have contracted the virus in New York, in Israel or on a flight between the two, Grotto said. Health authorities do not believed that she spread the virus to anyone on the flights.

She's unable to breathe on her own and on a respirator in the intensive care unit at Meir Medical Center in Kfar Saba, near Tel Aviv.

She developed a fever on March 31 and entered the hospital that same day.

Ongoing measles outbreaks in the United States and Israel started with parents who've chosen not to vaccinate their children, according to health authorities. Authorities believe that the flight attendant was vaccinated as a child, but the vaccine isn't perfect, and in her case, it didn't work.

"I knew this was going to happen sooner or later," said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University and an adviser to the US Centers for Disease Control and Prevention on vaccines. "We have the reintroduction of a serious viral infection with a population that's withholding the vaccine from their children, and now it's spreading beyond that population."

One dose of vaccine not optimal

Like many others of her generation around the world, the flight attendant, who has not been identified, received only one dose of the measles vaccine when she was a child.

It wasn't discovered until later that one dose is only about 93% effective. More recently -- in the United States, starting in 1989 -- children have been given two doses, which is about 97% effective, according to the CDC

It's not known why most people who get measles recover fully while others have devastating complications.

About 1 out of every 1,000 children who gets measles will develop encephalitis, according to the CDC. This can lead to convulsions and leave a child deaf or with an intellectual disability.

Additionally, 1 or 2 out of 1,000 US children who get measles will die from it. Worldwide, the illness is fatal in 1 or 2 out of every 100 children.

No fatalities have been reported in the United States from measles this year or last year. In Israel, a toddler and an elderly woman died last year of the disease. In the European Union, 35 people died of the disease in 2018, according to the European Centre for Disease Prevention and Control

Measles in Israel

There have been 3,920 cases of measles in Israel from March 2018 through April 11, said Grotto, who is also a professor of epidemiology and public health at Ben-Gurion University of the Negev.

In the United States , there have been fewer than 1,000 cases in about the same time period.

In Europe, there were 34,383 cases of measles based on data reported to the World Health Organization from April of last year to April of this year.

Ukraine had the highest number of cases in the past 12 months, with more than 72,000, followed by Madagascar and India with more than 69,000 and 60,000 cases, respectively. WHO warned that there are delays in reporting and that this data may be incomplete.

Ultra-Orthodox Jews, many from Israel, gathered in Uman, Ukraine in September on a religious pilgramage. Ukraine had more than 72,000 cases of measles last year.

Grotto said there was a surge of cases in Israel last fall when large numbers of ultra-Orthodox Israelis traveled to Ukraine on a religious pilgrimage during the Jewish New Year. Ukraine has had more than 72,000 measles cases this year, more than any other country, according to the World Health Organization

About 85% to 90% of Israeli measles cases have been among ultra-Orthodox Jews, Grotto said.

There's nothing in Judaism that teaches against vaccination; on the contrary, rabbis encourage vaccination in keeping with Jewish teachings on protecting your health and the health of others.

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Ultra-Orthodox Jews tend to have large families, and Grotto said those who don't vaccinate tend to have practical, not ideological, reasons.

"Sometimes, they vaccinate their first or second child, but with so many children, they don't always have time to vaccinate them all," he said. "They're not against vaccines. They have nothing against them ideologically."

To turn the tide in the outbreak, Israeli public health authorities have increased vaccine clinic hours, opened mobile clinics in religious neighborhoods and taken out ads in newspapers in religious communities.