Health officials are warning travelers who were in O’Hare Airport’s Terminal 5 between 4:20 and 7 p.m. Oct. 15 to monitor for measles symptoms after a vaccinated DuPage County resident tested positive following international travel.

The DuPage County Health Department announced Oct. 21 that a vaccinated county resident tested positive for measles after returning from international travel. The agency called it a rare breakthrough infection and identified Terminal 5 as the sole public exposure site, with the window running from 4:20 p.m. to 7 p.m. that day, according to the DuPage County Health Department.

The alert matters for Park Ridge and Niles residents who regularly use Terminal 5 for international flights. Anyone who was in the terminal during that window should monitor for symptoms through Nov. 5, a 21-day period after the potential exposure.

DCHD Executive Director Adam Forker said in a statement that "our team is actively working on following up with individuals who may have been potentially exposed."

Health officials said the overall public risk is low because most people are protected through vaccination. Vaccinated people who do contract measles tend to have milder illness and are less likely to spread the virus, the health department said. Two doses of the measles, mumps and rubella (MMR) vaccine are about 97% effective at preventing measles, according to the Chicago Department of Public Health.

Still, even vaccinated individuals who were in Terminal 5 during the exposure window should watch for symptoms. Those include fever, cough, runny nose, red and watery eyes, and a rash that usually appears three to five days after other symptoms begin, starting on the face and spreading downward.

Symptoms can take seven to 21 days to appear after exposure. Measles spreads through the air and can linger in a room up to two hours after an infected person leaves, according to the Chicago Department of Public Health.

What to do if you were there:

DuPage County residents who were in Terminal 5 during the exposure window should call DCHD at (630) 682-7400. Residents of other counties, including Cook County, should contact their local health department.

Anyone who develops symptoms should stay home, isolate, and call a doctor before visiting a medical office or emergency room. Calling ahead allows staff to take precautions to protect other patients.

Residents unsure of their vaccination status can check records through the Illinois Department of Public Health's Vax Verify portal or contact their doctor. People who have received two MMR doses or were born before 1957 are considered protected.

This is the second time Terminal 5 has been linked to a measles exposure alert in 2026. In late March, Cook County officials warned that a contagious individual had passed through the same terminal on March 24 and also visited two stores in Niles and an immediate care center in Mount Prospect before being identified, the Chicago Tribune reported, as cited by Hoodline.

Illinois has recorded 24 measles cases in 2026, CBS News Chicago reported, with about 79% occurring in people who were unvaccinated or whose vaccination status was unknown, according to WGN-TV. Nationally, the CDC had confirmed 4,410 cases as of Oct. 15, along with three deaths.

As of Oct. 23, no additional cases linked to the Terminal 5 exposure had been reported, according to Medical Daily. Park Ridge and Niles residents with questions about their exposure risk should contact the Cook County Department of Public Health.