Health officials in New York City have branded a single yeshiva - a Jewish Orthodox school - the driver of new measles cases.
The center in Williamsburg, Brooklyn, is linked to 21 of the 31 new cases of measles in the city, driving up the total number of cases to 121 since the outbreak began in October.
Orthodox Jewish communities have been hardest-hit by the spread.
Of the cases, 108 are in children under age 18 and eight people have been hospitalized.
The health department says Yeshiva Kehilath Yakov, and two other schools, disobeyed standing orders to keep out unvaccinated children.
City officials say if the yeshivas had simply complied, the outbreak might have been nearing its end instead of continuing to spread.
Yeshiva Kehilath Yakov, in Williamsburg, Brooklyn (pictured), has been linked to 21 of the 31 new measles cases identified in New York City after it disobeyed a standing order and allowed students who were unvaccinated to attend school
The Health Department has doubled down on efforts to promote vaccines in the Orthodox Jewish community, where there has traditionally been some resistance to immunization.
Thanks to the campaign, health officials say they were able to vaccinate an additional 7,000 children.
In December, the health department said students in certain zip codes in Borough Park and Williamsburg who were not up-to-date on the MMR (measles, mumps, rubella) vaccine would not be allowed to attend school.
Currently, about 1,800 students are not attending school because of the order.
But, according to WNYC, at least three schools disobeyed the order. Among them is the biggest culprit: Yeshiva Kehilath Yakov in Williamsburg.
'This yeshiva went out of compliance with the Health Department's exclusion order in mid-January, allowing an unvaccinated student who had measles but had not yet begun presenting symptoms [to attend school],' the department said in a press release.
Dr Demetre Daskalakis, Deputy Commissioner of Disease Control for the city, told WNYC that if the school had not violated the order, the outbreak would be close to over.
'The unvaccinated kid who got measles wouldn't have been in the school and there wouldn't have been a bunch of other [unvaccinated] students in the school and they wouldn't have gotten measles,' he said.
'So we would actually be at closer to what would be the tail end of this outbreak.'
Most people will recover from measles within one or two weeks, but complications can develop.
People most at risk of complications include teenagers and adults, babies
