A measles outbreak in Ohio is the worst observed in any U.S. state since the year 1996, health officials say.

Sixty-eight cases have been confirmed so far in an outbreak tied to unvaccinated Amish workers of the Christian Aid Ministries who traveled in the Philippines, currently in the grip of an epidemic of measles.

Around 20,000 cases in total have been counted in the Philippines.

The outbreak in Ohio is a significant concern to health professional who've largely considered the infectious and contagious disease as having been eliminated in this country since 2000.

Some of the early Ohio cases were misdiagnosed at first as being dengue fever, showing just how rare -- and thus unfamiliar -- the disease has become for American physicians, experts say.

"Because of the success of the measles vaccine, many clinicians have never seen measles and may not be able to recognize its features," Dr. Julia Sammons wrote in Annals of Internal Medicine.

New York and California have also experienced outbreaks of measles this year, with the national total set to pass 2013 numbers, which were three times the national average, the Centers for Disease Control and Prevention says.

The California outbreak was also tied to people who had visited the Philippines, officials said.

Small communities like the Amish, which have not been vaccinated either inadvertently or through choice, have been linked to previous outbreaks.

"The Amish who are family members and acquaintances of those who now have measles have been extremely cooperative in a willingness to get vaccinated," said Jackie Fletcher, nursing director for Ohio's Knox County Health Department. "And those who currently have measles have been staying home."

Many of the measles patients in Ohio are "really sick," she said, with coughs, fever and conjunctivitis.

In rare instances, measles can result in pneumonia and or even fatal brain infections.

"Although most of us don't realize it because it is so rare in our country, measles is a serious disease," CDC director Tom Frieden wrote on CNN's website in 2013. "Measles is so contagious that if one person has it, 9 of 10 people close to that person who are not immune will also become infected."