Health officials have warned of possible measles exposure at Chicago’s O’Hare International Airport, stressing the disease’s “highly contagious” nature.

Authorities said an infected traveler was in Terminal 1 on April 22 and 23 from 10 a.m. to 8 p.m., potentially exposing hundreds of passengers. The person had received one MMR vaccine dose, which offers partial protection, but later developed a rash on April 25. Testing confirmed measles, and the patient is now isolating while officials trace contacts.

Soon after, a second case was reported in the same Illinois county. Investigators have not determined whether the two infections are related.

Measles spreads through coughing and sneezing and can linger in the air for up to two hours. “Even brief contact in enclosed spaces like airports can lead to transmission,” officials warned.

Doctors emphasize vaccination as the best defense. Two doses of the MMR vaccine are about 97% effective. People unsure of their vaccination status should check records and get vaccinated if needed. Travelers who were in Terminal 1 during the exposure period should monitor for symptoms—fever, cough, runny nose, red eyes, and rash—and contact a doctor before visiting a clinic.