(Jeff Spielman/Getty Images)

Health officials are urging New Yorkers to get vaccinated amid a measles outbreak that has spread to the Lower East Side.

At least 25 New Yorkers - including 13 children - have contracted the virus, according to city Health Commissioner Mary Bassett. The outbreak, which emerged in northern Manhattan and the Bronx, has now spread downtown to the Lower East Side.

"Measles is highly contagious and can spread easily through the air," Bassett said in a statement today. "I urge New Yorkers to ensure all household members, including young children 12 months and older, are vaccinated."

The measles virus spreads through respiratory droplets - the mist exhaled during coughs and sneezes. When an unvaccinated person is exposed, there's a 90 percent chance they'll become infected. But a vaccine against measles, mumps and rubella known as the MMR vaccine can safely cut that risk by 95 percent, according to the New York City Department of Health and Mental Hygiene.

"Most people who receive the MMR vaccine do not have any side effects," the department said in a statement. "Some people experience mild side effects like fever, mild rash or swelling of glands in the cheeks or neck."

Measles usually causes a fever and rash that fade in a matter of days. But as many as one in three people experiences a dangerous complication, from pneumonia to brain infections. The virus can also cause miscarriages and premature labor in pregnant women, according to the Health Department.

At least four of the New York City measles cases have resulted in hospitalization, according to the Health Department.

Because measles is so contagious, health officials are urging New Yorkers to call a doctor before going to a busy clinic.

"If you suspect you have measles, please call your medical provider before seeking medical attention to avoid exposing others to the measles virus," Bassett said. "I strongly advise all urgent care centers, emergency departments, and clinics to educate your staff about measles and ensure that all patients who may have measles are immediately isolated."