Amid the largest measles outbreak in New York City since 1991, health officials Tuesday ordered nearly everyone in a heavily Orthodox Jewish neighborhood to be vaccinated or face fines.

• What to Know About the Measles Outbreak in NYC

Mayor Bill de Blasio declared a public-health emergency over a measles outbreak that has sickened nearly 300 people, including 246 children, since it began last October.

In an effort to contain its spread as the Passover holiday approaches, the mayor's office has required every person older than six months in the zip codes 11205, 11206, 11211, and 11249 — all in Williamsburg — to receive the measles vaccine. The order applies to anyone living, working, or going to school in the area.

The city's Department of Health and Mental Hygiene will review vaccination records of every person who may have been in contact with measles patients, and de Blasio says anyone who is not vaccinated could be fined $1,000. The city said Tuesday that it would help everyone covered by the order get the vaccine if they can't get it quickly through their regular medical provider.

The city has already ordered religious schools and day care programs serving Williamsburg to exclude unvaccinated students or risk being closed down.

The outbreak started in Williamsburg after a child who traveled to Israel contracted the virus in October last year. Nearly 300 people — mostly under the age of 18 — have become infected with measles, which, if left untreated, can have serious consequences — even death. City health officials say they've also been made aware that some parents are taking their children to "measles parties" in an effort to expose their children to the disease.

"This is the epicenter of a measles outbreak that is very, very troubling and must be dealt with immediately," de Blasio said at a news conference announcing the order Tuesday.

Officials acknowledged they have been battling an epidemic of public misinformation and a growing anti-vaxing movement, which falsely claims vaccines can harm children.

Dr. Herminia Palacio, the deputy mayor for Health and Human Services, stressed the measles vaccine is safe.

"I want to set the record straight: this vaccine is safe. This vaccine not only protects your child, but, as you heard from the mayor, it protects other people's children," Palacio said. "It protects members of the community who are vulnerable to disease and are unable to get vaccinated."

But officials are also confronting some who still believe measles is a harmless disease that children and adults can fight off, like one woman who did not want to be identified but said she had no plans to vaccinate her kids.

"My children had the measles and they recovered beautifully, so, no, that just proves that there is nothing to be concerned about," she said. "I believe that God created our bodies perfectly to be able to fight off these benign diseases."

Other officials, including Gov. Andrew Cuomo have echoed a similar concern, suggesting mandating vaccinations could soon be contested in the courts.

"It's a serious public health concern, but it's also a serious First Amendment issue and it is going to be a constitutional legal question," Cuomo said on WAMC Radio's The Roundtable with Alan Chartock.

Since officials recorded an uptick in October, the city has reached out to religious leaders and held meetings with top officials in the community in hopes of correcting the record on vaccination.

Rabbi David Niederman, executive director and president of the United Jewish Organizations of Williamsburg, Inc. said he's encouraged the community to get immunized.

"I've been telling the community: everyone has to be vaccinated. Do it to yourself, do it to your children, your family who you love so dear. And you are fooled by fake science and medical studies not to vaccinate. Trust the doctors in your community who say you have to vaccinate," Niederman said.

It is the first time in recent memory that the city has declared a public health emergency to require vaccination.

New Yorkers who want to know where to get the vaccines should call 311 or access the Department of Health website.