New York finds itself mired in the worst measles outbreak it's seen in recent memory, with 55 cases confirmed in the city since September. In Rockland and Orange counties, a further 112 people have been diagnosed.

"If you go back many decades ago when we weren't vaccinating, of course there were probably more outbreaks, but in my memory, I don't know of a measles outbreak that was this significant," New York State Commissioner of Health Howard Zucker told CNN. The city itself experienced a slightly larger outbreak in 2013, with 58 cases diagnosed, but statewide, this is the most aggressive spread health officials have seen in years. In 1991, more than 2,000 cases were reported to the city health department.

Unfortunately, the source of the outbreak appears to be unvaccinated children, who were exposed to the virus on a trip to Israel—which is also weathering an outbreak—early this fall. Those children, in turn, spread the disease to other unvaccinated kids with whom they came in contact. The disease has since clustered in Brooklyn's Orthodox Jewish communities: According to the NY Post, Borough Park has counted 32 cases, Williamsburg has had 21, Bensonhurst has seen one, as has Midwood/Marine Park. The vast majority of cases across the state have affected patients under the age of 18.

"We have immunized 13,000 children since this outbreak has begun," Zucker told CNN, but health officials are urging parents to vaccinate their children early: A Rockland County spokesperson told the Post that health officials now recommend getting the first measles, mumps, and rubella vaccine at six months, as opposed to a year, and the second as soon as 28 days later, rather than at age four.

"Parents who oppose vaccinations for measles and all other illnesses not only put their own children at risk, but endanger other children and families as well," City Council Health Chair Mark Levine said in a statement in early November. "The risk of measles affecting our New York communities is particularly acute in neighborhoods where international travel is common and frequent. I strongly urge all parents across the city to ensure their children are up to date on all AMA recommended vaccinations, including for the flu, as we enter the winter months."

Speaking of the flu, measles can initially look like that more common ailment: Coughing and sneezing spread the airborne infection, which ultimately announces itself with a distinctive, full-body rash. (Often in combination with things like fever and red, watery eyes.) Measles can cause brain swelling, and in rare cases, death.

Thanks to the development of the measles vaccine in the 1960s, medical professionals eliminated the disease in the U.S. by 2000. Now, the greatest measles risk comes with international travel, one compelling reason why it's important to vaccinate kids. How did we wind up in this current predicament? People haven't necessarily been doing that.

As Gizmodo points out, New York ranks among the 47 states that exempt parents from vaccinating their offspring if doing so challenges their religious beliefs. People of Orthodox Jewish faith are not uniformly hostile to vaccines, though. Rather, certain groups within Brooklyn's Orthodox communities subscribe to anti-vaxxer ideology, thanks in part to the efforts of organizations that have been spamming them with propaganda demonizing vaccines. And also, private schools haven't been doing an exemplary job of making sure students are up to date on their shots. So, parents, this one's on you: Vaccinate your kids! Thank you and good night.

Update: An earlier version of this story was headlined "NYC Faces Worst Measles Outbreak In Generations." In fact, 58 cases were diagnosed citywide in 2013, and in 1991, more than 2,000 cases of measles were reported to the city health department.