The Survivors need to hold out for a certain amount of time before they can be rescued by the authorities. This mode is focused on defending a position/area and simply surviving the threat.
The authorities have decided to launch an airstrike against the location so the survivors have to escape this location in time or they will perish along with the zombies. Their win condition is to escape – preferably uninfected, the zombies will of course have to stop them OR infect at least 50% of them.
The survivors are to gain control of the entire map so they need to kill all the zombies. This is obviously a more offensive task so, to not lose the entire building aspect they will need to prevent the zombies from entering so they will have to block entrances behind them or kill every last zombie on the map (including AI).
The game starts with all players being survivors and zombies are AI controlled. One of the survivors is infected, and it isn’t even clear to the infected survivor initially that he or she is infected. Only after the preparation phase does it slowly become apparent, and the infected survivor begins to cough. The goal of the infected survivor is to keep his infection secret, and eventually “turn” and kill the survivors, with the help of the AI zombie horde.