New York City health officials announced a new crackdown Monday on yeshivas in a Brooklyn neighborhood that are letting unvaccinated students attend school in the midst of a major measles crisis.

The Health Department had already ordered Jewish academies in certain zip codes to send home children who are not fully immunized, but some of the schools flouted the rules.

Now, the agency has extended the ban to all yeshivas in the Williamsburg neighborhood, warning that those who don’t could be “subject to fines or possible school closure.”

Since the city outbreak began in October, 285 people—most of them children—have contracted the highly contagious but preventable virus. Forty of the cases were from a single yeshiva that did not comply with city orders, say officials.

Twenty-one patients had to be hospitalized—contradicting the anti-vaxxer argument that measles is not a serious disease.

Officials have said vaccination rates in some Orthodox Jewish communities are lower due to religious exemptions from the shots—even though many religious leaders are urging their followers to get inoculated.

The outbreak in Brooklyn is the largest—but there is also a large outbreak in the Jewish community in Rockland County, New York, where a judge recently struck down an order banning unvaccinated kids from all public places. In addition, Michigan health authorities are trying to stem an outbreak in Oakland County, another Orthodox community, where the number of cases hit 41 this week.

Another outbreak in Washington State, centered in the Slavic community, stands at 74 cases with no new infections reported in several weeks.

Some of the outbreaks have been sparked by travel to and from Israel, where the disease has spread to thousands of people.

But health officials say a growing anti-vaxxer movement, fueled by debunked conspiracy theories, is also to blame for the comeback of a disease that was essentially eradicated in 2002.

The virus can live in the air for two hours after a single sneeze or cough, making it easy to spread in communities with lower vaccination rates or to babies who have not yet gotten the MMR shot.

New York health officials are particularly concerned that the Brooklyn outbreak is continuing to grow as Passover approaches and families plan gatherings.