[mr.pink]

In a closed air system, it seems this would be a problem.

Yup, the recirculated air can't be beat as a means of bacterial transmission. Some have argued the problem has gotten worse since the airlines banned smoking and hence cut back on the rate of air replenishment.

The airline industry needs this, at this time, like a hole in the head.

[DoughtyOne]

Yes you are right, they sure don't.