The Health Department has now confirmed 25 cases of measles since February 5th. Residents in Upper Manhattan, the Bronx, Brooklyn, and now the Lower East Side have contracted the virus, and health officials are urging residents to get vaccinated if they have not already.

This week, the DOH confirmed three new cases of measles on the Lower East Side. Now, a total of 12 children and 13 adults have been infected with the disease, which was nearly eradicated a few years ago thanks to vaccinations. "I urge New Yorkers to ensure all household members, including young children 12 months and older, are vaccinated," Health Commissioner Mary T. Bassett said in a statement. "Measles is highly contagious and can spread easily through the air."

It's suspected that the outbreak started in city hospitals and medical facilities, and may have been allowed to worsen by doctors and nurses who didn't immediately recognize the symptoms. "I think what we’re seeing is a pocket of infection here in New York City that has emerged, essentially from a group of patients who have been exposed,” Dr. Robert Glatter, an emergency medicine physician at Lenox Hill Hospital, told CBS News. “It’s not clear how."

Though measles all but disappeared thanks to the advent of the measles, mumps and rubella (MMR) vaccine, the disease may have been brought here from overseas, then spread among people who either did not receive the vaccination or were too young to be properly vaccinated. Eight of the 12 children who contracted measles were too young to be vaccinated, and one had only received one of the two recommended vaccines. Two were not vaccinated, likely due to recent (and debunked) theories that vaccination leads to autism in children.

According to the Department of Health's website, symptoms of measles include high fever, cough, runny nose, sore throat and red, watery eyes, as well as little white spots on the gums and inside of the cheeks, and a rash of red spots on the face and body. "If you suspect you have measles, please call your medical provider before seeking medical attention to avoid exposing others to the measles virus," Bassett warns.