This year is on track to be the worst for measles in more than a decade, according to new numbers released Thursday by the U.S. Centers for Disease Control and Prevention. And people who refuse to vaccinate their children are behind the increasing number of outbreaks, health officials say.

The U.S. had 159 cases of measles from Jan. 1 through Aug. 24, according to the CDC. If that trend continues, there will be more cases in 2013 than in any year since 1996, when about 500 cases were reported. The number would also surpass that of 2011, when there were 222 cases.

Measles cases in the United States numbered in the hundreds of thousands before the advent of vaccination, and dropped dramatically throughout the 1960s. The disease was thought to have been eradicated in 2000, but the numbers have recently crept back up, largely because of visitors from countries where measles is common and because of vaccine objectors within the United States. Nearly two-thirds of the reported cases happened in three outbreaks in communities where many people don't vaccinate their children for religious or philosophical reasons.

"This is very bad. This is horrible," said Dr. Buddy Creech, a pediatric infectious disease expert at Vanderbilt University who was on a telephone briefing with the CDC Thursday morning. "The complications of measles are not to be toyed with, and they're not altogether rare."

According to the CDC, one to three out of every 1,000 children in the United States who get measles will die from the disease, even with the best of care. Even if complications such as pneumonia and encephalitis aren't deadly, they can make children very sick; in 2011, nearly 40% of children under the age of 5 who got measles had to be treated in the hospital.

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