Experience shows us that transmission becomes widespread within the passenger cabin involving all sections when the ventilation system is not working -- as evidenced by an influenza outbreak in 1979 involving passengers being kept aboard grounded aircraft with an inoperative ventilation system.

CNN: Commentary: Flu and your health on a plane

Ventilation can involve either 100 percent fresh air in which outside air enters and leaves the cabin in a single pass or a system in which various fractions of air are recirculated from the aircraft cabin and mixed with fresh air.

CNN: Commentary: Flu and your health on a plane