Board Thread:Off Topic/@comment-26397825-20160528034745/@comment-24534644-20160530003423

The haze operates under the rules of regular gases, and can be blown away by gusts of wind. The haze also will quickly dispel if it gets far enough away from Rosie, and if she deactivates her Semblance, though it isn't instantaneous; it will linger for a brief amount of time, just like any gas that suddenly gets its source cut off.

As for exposure, people who enter the haze will not "instantly contract Black Death, Polio, and Ebola", as you said. The effect is gradual, for one, and targets to not contract actual diseases. They merely experience symptoms similar to sickness and plagues. The effects escalate the longer a target stays within the AOE though, eventually reaching deadly levels if a target without Aura protection stays within it.