The Center for Disease Control and Prevention confirmed this week that at least 16 cases of the measles are present around the Knox County village of Danville.
The outbreak was first discovered earlier this week when four members of a local Amish community returned from a trip to the Philippines.
Each member was unvaccinated and, upon offering humanitarian aid to typhoon victims in the Pacific country, contracted the virus.
The four returning Amish are believed to have infected more than ten others, ranging in age from to 2 to 48, Palm added, although definitive tests are still pending.
Small Amish communities quietly exist in secluded areas around the US, but perhaps most prominently in the farmlands of Ohio and Pennsylvania.
A deeply religious community of just under 300,000, the Amish are known for their simple dress, long beards, and traveling in horse-carriages along rural roadways.
They reject the notion of modern technology, believing that traditional work, like their reliance on Pennsylvania Dutch and community relations, is one of the benchmarks of religious devotion.
The Amish do not specifically prohibit vaccination, although a 2011 study from the Journal of the American Academy of Pediatrics found that only a fraction of Amish children have received immunizations compared to the general public.