Health officials have issued an urgent warning after a possible measles exposure at Chicago O’Hare International Airport. An infected adult, who had received one dose of the MMR vaccine, was contagious while spending two days in Terminal 1 on April 22 and 23 between 10 a.m. and 8 p.m.

Measles is “one of the most contagious viruses known” and can remain in the air for up to two hours after an infected person leaves. Anyone passing through the terminal during those times may have been at risk.

By April 25, the traveler developed the measles rash and has since isolated at home. Officials stressed that “early isolation is key to preventing additional spread.”

Days later, a second case appeared in the same Illinois county. The adult patient sought care on April 28 and was immediately isolated at the hospital, while health authorities began tracing contacts.

Measles can lead to pneumonia, brain swelling, and even death, especially in children, pregnant women, and those with weak immune systems. Officials urge anyone at O’Hare during the exposure to check vaccination records and, if unvaccinated or unsure, contact a healthcare provider immediately.