In this country, the biggest outbreak is centered in the Amish community in Ohio, where many of the residents are unvaccinated, the CDC reported. In Virginia, two cases were confirmed earlier this month.

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"This is a wake-up call for travelers and parents to make sure vaccinations are up to date," Schuchat said.

"Measles vaccine is very safe and effective and measles can be serious," she added. "It's very infectious." Forty-three of the people in this country who have come down with measles required hospitalization, most often for pneumonia, she said. No deaths have been reported here.

Measles is a highly contagious respiratory disease that generally affects young children, causing fever, a runny nose, a cough and a distinctive rash all over the body. This year, however, more than half the people who have come down with it are 20 years old or older, according to CDC data.

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About one in 10 children also gets an ear infection and one in 20 comes down with pneumonia. A person with measles is contagious as long as four days before the symptoms are apparent. Parents and even physicians who haven't seen measles in years may be unaware of the early warning signs.

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The largest number of confirmed cases of measles since the infection was eliminated in the United States in 2000 occurred in 2011, when 220 were recorded. The CDC hasn't seen this many cases so early in the year since 1994, when 764 people were infected by this time, Schuchat said.

In the past 20 years, a concerted public health campaign, especially among lower-income families, has made measles outbreaks rare in the United States. But an estimated 20 million people are infected in Europe, Asia, Africa and elsewhere each year, and 122,000 of them die.

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In the United States, the number of people who choose not to be immunized for religious, philosophical or personal reasons has begun to become a public health problem, Schuchat said. Others are unaware of, or unable to get, vaccinations before they arrive in the United States. A small number of adults can lose their immunity over time and may need to be re-vaccinated.

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Authorities aren't sure how the Amish community in Ohio contracted the disease, but Schuchat said they believe that people traveling to conduct faith-based work abroad are involved. According to the CDC, 40 importations of the infection were attributed to unvaccinated U.S. travelers returning from abroad.

Steven Nolt, a history professor at Goshen College in Indiana who has written about the Amish, said some groups do travel to places such as Kenya, Ukraine and Central America to do mission or relief work. He said many Amish do vaccinate themselves and their chidren, but others refuse.

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Some, Nolt said, have a "more traditional, conservative, old-fashioned way of life and set of sensibilities that views medicine as something that is used to heal or cure, rather than to prevent" disease. Others have a "theologically informed...sense that we should place our trust in God and not in vaccines."

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Schuchat urged anyone who isn't sure whether his or her immunizations are up to date to get another dose of the vaccine, especially if traveling to places like the Philippines or doing health care work.

Though the vaccine generally isn't given to children before the first birthday, infants traveling abroad can be inoculated with one dose as young as six months, Schuchat said. The vaccine is generally administered in two doses a few years apart. People whose immune systems are suppressed and pregnant women should not receive the vaccine, she said.

Adults born before 1957 likely had the measles and should be immune, she added. The vaccine became available in 1963.