New York City health officials are warning New Yorkers to get measles shots, if they haven’t already, after identifying 16 cases of measles in Manhattan and the Bronx, according to CBS News.

Nine of the cases are children, and seven are adults. Four people have been hospitalized as a result of the outbreak, according to the New York City Department of Health and Mental Hygiene.

Measles, also called rubeola, is a highly contagious respiratory disease caused by a virus. Symptoms of the disease include fever, runny nose, cough and a rash all over the body. Measles virus normally grows in the cells that line the back of the throat and lungs. Measles spreads through the air by breathing, coughing or sneezing. According to CDC, any child who is exposed to the disease and is not immune will probably get it.

About 30 percent of measles cases lead to serious complications including pneumonia, ear infections, diarrhea, brain inflammation, hospitalization and even death.

The health department is urging anyone who has not been vaccinated to get an MMR vaccine, particularly children. Adults who are unsure whether or not they’ve received the shot can either be vaccinated again or take a blood test to see if they are immune. Babies should get the vaccine at 12 months of age. Two doses are required for full protection, with the second shot administered at 4 to 6 years old. Side effects of the medication are typically mild and include soreness at the site of the shot.

The CDC recently issued a warning that once-eliminated measles had come back in elevated numbers — about 175 cases in 2013, compared to the typical 60 each year. Anti-vaccination beliefs were suspected to be behind these disease rate increases.

While measles is almost gone from the United States, it still kills an estimated 164,000 people each year around the world. Measles can also cause miscarriages and premature birth.

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