In 2014, a 19-month-old child connecting on a flight from India passed measles on to a man at a Chicago airport, even though they didn’t fly on the same plane. The man had arrived from Minnesota and disembarked at the same gate where the toddler was waiting to board the return flight.

“Although transmission could have occurred anywhere in the airport where the child and the adult shared airspace, it most likely occurred in the gate area,” the C.D.C. wrote in its report on the incident. It did not identify the airport.

The child had received a vaccination against measles at 12 months, but not the recommended booster. The man’s immunization history was unknown. No notification system is set up to alert at-risk travelers of potential measles exposure in airports.

On planes, however, the law requires that the C.D.C. be contacted when health officials identify someone with the measles as having been contagious while onboard. In those instances, health officials use the flight manifest to contact those seated within two rows of the infected passenger. They also notify anyone on the plane with a lap child, on the premise that they are more likely to wander around the aircraft, the C.D.C. said.

If an unvaccinated person can be notified quickly, then within 72 hours of exposure they can receive a dose of the MMR vaccine, which might prevent them from getting the disease or lessen its severity. If notification comes within six days, or if the person is not eligible to receive the vaccine and is notified within that time frame, then they can receive a shot of immunoglobulin, which are pre-made antibodies.