Measles is one of the most contagious viruses on earth—a pathogen so easily transmitted that it can linger in the air for up to two hours after an infected person leaves the area. Public health officials have long warned that airports, crowded terminals, and enclosed aircraft cabins pose unique risks when infections emerge. This past weekend, those warnings became a very real public-health emergency at St. Louis Lambert International Airport, which quickly went into high-alert mode after a passenger traveling through the facility tested positive for measles, prompting immediate exposure warnings for thousands of travelers and staff.