New York City Mayor Bill de Blasio on Tuesday declared a public health emergency in a heavily Orthodox Jewish section of Brooklyn amid a growing measles outbreak.As part of the declaration, people living in select zip codes of Brooklyn's Williamsburg neighborhood who have not been vaccinated against measles and may have been exposed to the highly contagious virus will now be required to get the measles-mumps-rubella (MMR) vaccine. Those who don't comply could be subject to a $1,000 fine."There's no question that vaccines are safe, effective and life-saving," de Blasio said in a statement. "I urge everyone, especially those in affected areas, to get their MMR vaccines to protect their children, families and communities."A total of 285 cases of measles have been confirmed in New York City's Orthodox Jewish community since the outbreak began last October. The vast majority of cases involved children under 18 who were not vaccinated or who had not received the required number of doses of the MMR vaccine, according to the New York City Department of Health and Mental Hygiene.Although no one has died, 21 patients have been hospitalized, including five who were admitted to an intensive care unit.The declaration comes after the city's health commissioner, Dr. Oxiris Barbot, threatened yeshivas in the affected areas of Williamsburg with violations and possible closure if non-vaccinated students are allowed to attend while the outbreak is ongoing.One yeshiva in Williamsburg fell out of compliance with Barbot's mandatory exclusions of non-vaccinated students in January. Forty measles cases have since been linked to that single yeshiva, according to the New York City Department of Health and Mental Hygiene."As a pediatrician, I know the MMR vaccine is safe and effective," Barbot said in a statement Tuesday. "This outbreak is being fueled by a small group of anti-vaxxers in these neighborhoods. They have been spreading dangerous misinformation based on fake science."Measles is an airborne virus that easily spreads through coughing and sneezing. It causes fever, runny nose, cough, red eyes and sore throat, followed by a rash. An infected person will start being contagious four days before a rash appears and will stop being contagious four days after rash onset, according to the Centers for Disease Control and Prevention.Measles is rare in the United States because of high vaccination rates, but the disease is still common in other countries. The ongoing outbreak in Brooklyn began when an unvaccinated child got infected during a trip to Israel, where a large outbreak is occurring, according to the New York City Department of Health and Mental Hygiene."Measles is a dangerous, potentially deadly disease that can easily be prevented with vaccine," New York City's deputy mayor for health and human services, Dr. Herminia Palacio, said in a statement Tuesday. "When people choose not to get their children vaccinated, they are putting their children and others -- such as pregnant women, people on chemotherapy, and the elderly -- at risk of contracting measles. The city has worked aggressively to end this outbreak, and today's declaration of a public health emergency and new vaccine mandate, in combination with the blanket commissioner's orders for yeshivas, ensures we are using every tool to protect New Yorkers."