The DuPage County Health Department called the infection a rare breakthrough. It said measles can occur after vaccination, particularly following intense or prolonged exposure, but vaccinated people who become ill typically have milder cases, face a lower risk of complications and are less likely to spread the virus.

Anyone who was in Terminal 5 on Oct. 15 between 4:20 and 7 p.m. and lives in DuPage County is asked to call the health department at 630-682-7400. People who were at the airport during that time but live elsewhere should contact their own county health department. The department said it is working to reach people who may have been exposed and described the overall public risk as low.

“Vaccination continues to provide strong protection, even in a rare breakthrough case,” health department executive director Adam Forker said in a statement.

The department considers people who have received two doses of the measles, mumps and rubella (MMR) vaccine, as well as those born before 1957, protected. Anyone unsure of their vaccination history should check with a doctor. People who may have been exposed should watch for fever, rash, cough, runny nose or red, watery eyes, whether vaccinated or not.

Illinois had recorded 24 measles cases in 2026 as of the announcement. The state’s case count included 11 in Douglas County, five in Moultrie, three in Madison, two in Kane and one each in Cook, DeKalb and DuPage. State data cited by the health department showed that 79% of cases were in people who were unvaccinated or whose vaccination status was unknown.

Measles spreads easily, making timely exposure notifications important even when a case occurs in a vaccinated person. DuPage health officials said most county residents are vaccinated and urged anyone with questions about a possible exposure to contact public health officials.