Researchers believe this year will be the worst for cases of the measles in more than a decade, CNN reported.

According to data released Thursday by the Center for Disease Control and Prevention (CDC), there have been 159 cases of the measles from Jan. 1 to Aug. 24 of this year. That number is likely to surpass the 222 cases reported in 2011, but would not likely reach the 500-or-so cases in 1996.

Measles have decreased dramatically since the arrival of a vaccine in the 1960s; beforehand, yearly cases could push into the hundreds of thousands.

In 2000, some experts though it was eradicated, but cases crept back due to visitors from foreign countries where the disease was common. Also the cause for outbreaks are pockets of people in communities who refuse to vaccinate their children and end up causing others in the community to catch the virus.

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

The CDC reports that one to three out of every 1,000 children with measles in the U.S. will die from it, even with top-flight care. In 2011, 40 percent of children under five years old with measles had to be treated in the hospital.

One of Creech's main concerns is that the younger generation of doctors will not recognize the early warning signs of measles.

"Many young pediatricians might not know what measles looks like," he said, noting that there have only been small pockets of the disease since 2000.

92 percent of people in the U.S. who came down with measles this year were not vaccinated or had an unknown vaccination status. The largest outbreak was in New York in a community where 58 people were infected because they refused to get vaccinated for religious reasons.

"I hope that those who are vaccine hesitant or vaccine avoidant realize there are consequences to their actions," Creech said. "None of us lives in isolation."