Multiple incoming reports show that the 6th zone is viraemic! Once the survivors reach it, they contract an incurable illness that prevents them from clearing the zone and completing the game!
The lab tests have detected several vulnerabilities of the virus and resulted in the creating an antidote. Within a couple of hours, the antidote will be air-sprayed over the 6th zone.
Upon the air-spraying completion, the survivors will be able to clear the zone!
Over.