A measles outbreak in Ohio has infected 68 people with the virus, extending the United States' 18-year high of measles cases. 

The 18-year high is confounding health experts, who thought the disease was eradicated in the U.S. in 2000.

The outbreak began when a group from Christian Aid Ministries went on a mission to the Philippines earlier this year and brought back the virus. The Philippines is suffering a severe measles epidemic with at least 20,000 cases confirmed and suspected, CNN reported. 

Four people on the trip became infected with the virus, and the disease spread to 62 other individuals in the Amish community in Knox County, Ohio. The county has 40 confirmed cases total. 

The first few cases were initially misdiagnosed as dengue fever as measles cases are far and few between in the United States. 

"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 a commentary published in April. 

Nearly 800 people have been immunized in Ohio since the outbreak. 

"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, director of nursing for the Knox County Health Department. "And those who currently have measles have been staying home."

California also has an usually high number of measles cases necause of people visiting the Philippines. 

Visitors have been contracting the disease then bringing it back to the states and potentially infecting people who have not been vaccinated. 

There were 129 cases of measles in the United States between Jan. 1 and April 18, according to the Centers for Disease Control and Prevention, which is the highest number of cases recorded since 1996. 

Symptoms of the disease include fever, cough, rash and conjunctivitis. In some rare cases, measles can cause pneumonia or brain infections, which can lead to death. 

"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 said.