The Rockland County, N.Y., woman hadn’t told her obstetrician that she had a fever and rash, two key signs of a measles infection. A member of the Orthodox Jewish community there, she went into premature labor at 34 weeks, possibly as a result of the infection. Her baby was born with measles and spent his first 10 days in the neonatal intensive care unit.

The infant is home now, but “we don’t know how this baby will do,” said Dr. Patricia Schnabel Ruppert, the health commissioner for Rockland County. When young children contract measles, they face a heightened risk of complications from the disease, including seizures or hearing and vision problems down the road.

The measles case Ruppert described is just one of many. New York state’s outbreaks, which began last October, have gone on longer and infected more people than any other current outbreak nationwide. More than 275 cases of the disease have been confirmed statewide through the first week of March, primarily in the New York City borough of Brooklyn and in Rockland County towns northwest of the city.

That total makes up about half of the 578 confirmed cases in 11 states that were reported nationwide by the federal Centers for Disease Control and Prevention from January 2018 through the end of last month. Washington state, with 76 cases by the end of February, has the second-highest number of cases.

Measles cases in New York have been concentrated among children from Orthodox Jewish families, many of whom attend religious schools where vaccination rates may have been below the 95 percent threshold considered necessary to maintain immunity. The outbreaks began when unvaccinated travelers returned from Israel, where an outbreak persists, and spread the disease here.

Besides geographic proximity, cultural identity may contribute to an outbreak taking hold in the close-knit Orthodox community — a feeling that their worldview is not in keeping with modern secular society, said Samuel Heilman, a Queens College sociology professor who has authored several books about Orthodox Jews.

“It’s about a view that we have our ways and they have their ways,” he said.

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Although some Orthodox Jews claim that vaccinations are against Jewish law, that’s not correct, said Dr. Aaron Glatt, who is also a rabbi and chairman of the department of medicine at South Nassau Communities Hospital on Long Island. “There’s not a single opinion that says vaccination is against Jewish law,” he said.

As public health officials and health care providers battle to get the outbreaks under control, one of their biggest challenges is communicating to people that measles is a menacing disease to be taken seriously.

“People don’t want to get vaccines because they don’t think they need them,” said Dr. Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia.

The public may have grown complacent. Before the vaccine program began in the United States in 1963, as many as 4 million people became infected every year. Nearly 50,000 were hospitalized and up to 500 people died annually. By 2000, measles was a disease that public health officials said was essentially eradicated in the U.S., thanks to a comprehensive vaccine program that reduced the number of cases by 99 percent.

But measles has crept back in recent years, in part because of fears fanned by anti-vaccine activists who have claimed, without evidence, that vaccines cause a variety of problems, including autism.

The measles virus is still a problem in some other countries, and unvaccinated people may bring the virus back with them and infect others.

The virus is exquisitely contagious. If an infected person coughs or talks, droplets can remain in the air or land on a surface and cause infection for hours. Ninety percent of people who are exposed and susceptible will become infected. While a fever and red rash that spreads from the face down along the body are common symptoms, side effects can be serious and even lethal, especially for young children and people with compromised immune systems.

In an effort to contain New York’s outbreaks, Ruppert initially prohibited 6,000 children at 60 mostly religious schools and day care centers in Rockland County from attending class because they hadn’t been vaccinated. As more children have been vaccinated and the school vaccination rates have reached 95 percent, those numbers have dropped. But about 3,800 students at 35 schools are still excluded from attending.

In Brooklyn, 1,800 students at 140 schools were originally affected, said Dr. Jane Zucker, assistant commissioner for the Bureau of Immunization at the New York City Health Department. Those numbers have declined somewhat as well.

Since the outbreaks began, Rockland County health care providers have administered more than 16,000 vaccines, while New York City has provided more than 7,000 shots.

The measles, mumps and rubella vaccine is given in two doses, the first when a child is between 12 and 15 months old, and the second between ages 4 and 6. It is 97 percent effective in preventing the disease.

In New York and nearly every other state, children may be exempted from vaccination requirements for medical or religious reasons. Seventeen states allow parents not to vaccinate their children for philosophical reasons, according to the National Conference of State Legislatures, but New York is not one of them.

Concerns about contracting measles aren’t limited to people who are part of the Orthodox Jewish community. Heilman said that one of his daughters-in-law likes to shop at a kosher supermarket in Rockland County, not far from her home in suburban White Plains. But with an 11-month-old daughter who hasn’t yet been vaccinated, she doesn’t want to take any risks. As long as the threat of measles continues, his daughter-in-law is shopping elsewhere, Heilman said.