The United States hasn't seen so many measles cases since 1994, with 288 cases of the highly contagious respiratory disease reported to the Centers for Disease Control and Prevention so far this year — about double the number seen last year.

“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 United States and spread to others in communities where many people are not vaccinated,” said Dr. Anne Schuchat, assistant surgeon general and director of CDC's National Center for Immunizations and Respiratory Diseases, in a press release.

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The vast majority (97 percent) of the U.S. infections were imported from at least 18 countries, including the Philippines, where there are about 26,014 suspected cases, over 6,000 confirmed cases, and at least 41 deaths.

Among the infected U.S. residents who were not vaccinated (90 percent), most (85 percent) chose not to vaccinate for religious, philosophical or personal reasons.

The U.S. cases tend to be clustered. In Ohio, for example, unvaccinated Amish missionaries who traveled to the Philippines may have spread the disease to at least 138 people.

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“We believe people are still getting exposed,” Melanie Amado, public information officer for the Ohio Department of Health told CNN earlier this week, because the incubation period for measles is up to 21 days.

Other affected areas include California with 60 cases and New York City with 26 cases; 17 states overall have reported measles in patients ranging from babies to 65-year-olds.

“We also think of measles as a childhood disease, but today’s report reminds us that there are many adults who never have received the childhood vaccines but are still traveling the world,” Schuchat said. “People may not think of MMR [measles, mumps, rubella] as a travel vaccine, the way they think of typhoid or yellow fever vaccines, but acquiring measles while traveling is likely if you have not actually been vaccinated.”

Photo: Sebastian Kaulitzki/Science Photo Library/Corbis