As some of these cells succumb to the measles virus, they break apart and slough off into the airway, triggering the need to cough. "Measles basically uses the trachea as a trampoline," says Cattaneo. "The cough bounces it out." Because the trachea is the last stop before your mouth and the outside world, this exit strategy is vastly more efficient than that of other respiratory viruses, like influenza, which replicate deep in the lungs. For those infections, any viral particles expelled at the speed of a cough have to sneak past a maze of mucous membranes in the linings of the lungs, which can catch or at least slow down their escape. But with measles, the majority of the crippled cells along with their viral invaders escape easily to the outside world, producing an infectious cloud that can stay suspended for up to two hours.