(CNN) -- Sixty-eight and counting.
An outbreak of measles in Ohio has infected 68 people, adding to what is already an 18-year high of measles cases in the United States.
That's disturbing news for health professionals, who considered the contagious infectious disease eliminated in the United States in 2000.
The outbreak in Ohio began with a group from Christian Aid Ministries, who went on a mission trip to the Philippines earlier this year, health officials say. Philippines is experiencing a very large measles outbreak; at least 20,000 confirmed and suspected cases have been reported in the Asian nation.
Four people who were on the mission trip became infected, according to Pam Palm, the public information officer for Knox County Health Department, and the disease has since spread to 62 others in the Amish community. Knox County has 40 cases.
Palm said the first few cases were initially misdiagnosed as dengue fever, a testament to how few cases of measles doctors usually see.
"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 in Annals of Internal Medicine.
Ohio health officials have immunized nearly 800 people with the MMR (measles, mumps, and rubella) vaccine since the outbreak began.
"The Amish who are family members and acquaintances of those who now have measles have been extremely cooperative in a willingness to get vaccinated," Jackie Fletcher, director of nursing for the Knox County Health Department, said in a statement. "And those who currently have measles have been staying home."
California, another state reporting a high number of measles cases this year, said its outbreak also resulted from people visiting the Philippines.
Visitors may pick up the disease and bring it back to the United States, potentially infecting those who cannot be vaccinated against the measles because they are too young, for example, or who have intentionally remained unvaccinated.
Data released by the Centers for Disease Control and Prevention on April 24 found 129 cases of measles in the United States between January 1 to April 18. That's the highest number of cases recorded for the period since 1996. Some of the Ohio cases were recorded after that reporting period -- meaning the total now is undoubtedly higher.
Fletcher said many of the measles patients her staff are seeing are "really sick." Symptoms usually include fever, cough and conjunctivitis, along with a rash. In rare cases, measles can lead to pneumonia and brain infections, which can be fatal.
"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 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."
CNN's Jacque Wilson, Trish Henry, Tina Burnside and Dave Alsup contributed to this story.