Health officials are warning travelers about a possible measles exposure at Chicago O’Hare International Airport.

An infected adult spent time in Terminal 1, potentially exposing many people to what authorities describe as “the world’s most infectious disease.” The individual was at the airport — “the world’s fourth busiest in terms of passenger numbers” — on April 22 and April 23 between 10 a.m. and 8 p.m.

Although the person had received one prior dose of the MMR vaccine, they developed a rash on April 25. Testing later confirmed measles. Since the diagnosis, the individual “have been isolating home.”

A second case was identified days later in another adult from the same Illinois county. Their vaccination status is unknown. On April 28, they checked into a local hospital for treatment, where medical staff “quickly isolated them” to prevent further spread.

Health authorities are urging anyone who may have been at O’Hare’s Terminal 1 during the listed times to contact their healthcare provider and monitor for symptoms. Early symptoms can include fever, cough, runny nose, red eyes, and a rash that typically appears several days later.

Officials stress the importance of vaccination and prompt medical guidance. This incident is part of “the latest in a string of measles scares to rock America’s major airports and surrounding neighborhoods,” highlighting how quickly the virus can spread in crowded travel hubs.

Travelers are encouraged to review their vaccination records and seek advice if they believe they were exposed.