Major U.S. airport on red alert as passenger is diagnosed with world's most infectious disease after receiving MMR vaccine | 1 May 2025 | Health officials are sounding the alarm over a potential measles exposure at Chicago O'Hare International Airport. An infected adult was mainly confined to the airport's Terminal 1 area, potentially exposing hundreds of others to the world's most infectious disease. The person was at the airport -- the world's fourth busiest in terms of passenger numbers -- on April 22 and April 23 between 10am and 8pm both days. They had had one prior dose of the MMR vaccine, but they went on to develop a rash on April 25 and tests confirmed they had contracted measles. Another case was identified days later in an adult living in the same county in Illinois.