An outbreak of mumps in Brooklyn, New York and Lakewood, New Jersey proves that even diseases that have largely been eradicated can still find a foothold and spread. Here’s what you need to know about the symptoms and complications of mumps.

According to New York City’s NPR station, a Brooklyn-based child went to summer camp in England—where vaccination rates are lower—and brought the mumps back home. Now at least 57 children in Brooklyn, and about 30 people in Lakewood, New Jersey have contracted the disease. Most of those in New Jersey who got the mumps had been fully vaccinated, according to local reports. And a reported 75 percent of the Brooklyn children infected had also been vaccinated.

Studies show that 9 in 10 people vaccinated against the mumps will develop antibodies, and therefore immunity, against the disease, but that means that 10 percent won’t. And those that don’t get vaccinated—like the reported 25 percent of cases in Brooklyn—have little or no protection against the disease. The lower the vaccination rate, the easier it is for the disease to get a foothold and spread—even to those who have been vaccinated. But when more people are vaccinated, it’s harder for the virus to gain traction. That’s called “herd immunity,” and it’s why vaccine programs are more effective when a greater proportion of the population takes part.