Health officials in Mercer County reported a laboratory-confirmed measles case involving a county resident, according to the Mercer County Health District.
The district said it believes the person was unlikely to have been in contact with the general public while able to spread the virus.
According to the department, measles symptoms generally emerge 10 to 12 days after a person is exposed. It also said a rash typically develops three to five days after other symptoms appear.
The health district placed the scale of the outbreak at about 300 confirmed cases in Ohio during 2026, with approximately 100 confirmed cases in neighboring Indiana.
Further details about the patient, including age, vaccination history or how the infection was acquired, were not included in the information released. The department's assessment that the public was unlikely to have been exposed during the infectious period is a determination by the agency rather than independently verified information.