With 288 cases of measles reported in 18 states in the U.S. in the first five months of this year, the Centers for Disease Control (CDC) are urging people to get vaccinated, especially those who are planning international travel this summer.

This is the highest number of measles in the U.S. since 2000, when the disease was eliminated, and the largest number of measles cases in the U.S. for the first five months of a year since 1994. Almost all the measles cases this year have been associated with international travel by unvaccinated people.

People who were infected range in age from two weeks to 65 years, with over half adults age 20 or older.

Ohio had the largest outbreak, with at least 138 cases, followed by California, with 60 cases, and New York City, where 26 cases were reported.

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Measles is a highly contagious respiratory disease caused by a virus. Measles is still common in many parts of the world, including countries in Europe, Asia, the Pacific, and Africa. Worldwide, an estimated 20 million people get measles and 122,000 die from the disease each year.

Measles causes fever, runny nose, cough, and a rash all over the body. About one out of 10 children with measles also gets an ear infection, and up to one out of 20 gets pneumonia. For every 1,000 children who get measles, one or two will die.

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Dr. Anne Schuchat, assistant surgeon general and director of the CDC’s National Center for Immunizations and Respiratory Diseases, said in a press statement, “The current increase in measles cases is being driven by unvaccinated people, primarily U.S. residents who got measles in other countries, brought the virus back to the U.S., and spread it to others in communities where many people are not vaccinated. Many of the clusters in the U.S. began following travel to the Philippines, where a large outbreak has been occurring since October 2013.”

Of the 288 cases, 280 (97 percent) were associated with importations from at least 18 countries. More than one in seven cases has led to hospitalization. Ninety percent of all measles cases in the U.S. were in people who were not vaccinated or whose vaccination status was unknown. Among the U.S. residents who were not vaccinated, 85 percent were for religious, philosophical, or personal reasons, said the CDC.

Infants and young children are at high risk of getting a serious case of measles. The CDC recommend two doses of measles, mumps, and rubella (MMR) vaccine for everyone, starting at age 12 months, but for people traveling internationally, the CDC advise all U.S. residents older than 6 months receive MMR vaccine, if needed, prior to departure.

Schuchat said many U.S. healthcare providers have never seen or treated a patient with measles because of the nation’s robust vaccination efforts and rapid response to outbreaks.

Before the measles vaccination program started in 1963, about 3 to 4 million people got measles each year in the U.S. Of those people, 400 to 500 died, 48,000 were hospitalized, and 1,000 developed chronic disability from measles encephalitis.

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People should be evaluated for measles if they have fever and rash along with cough, runny nose, or pink eye, especially if the patient is unvaccinated and recently traveled internationally, or was exposed to someone else who has measles or recently traveled.

If healthcare providers believe a patient has measles, they should immediately isolate the patient to help prevent the disease from spreading, according to the CDC. They should also immediately report the case to their local health department and collect specimens for serology and viral testing.