My friend was tellling me that all throughout the campaing hints of the "zombies invasion" were dropped. I didn't notice anything except for obviousey the part after you beat campaign alltogether. Is he just bull-blam!- me or did I really just miss this part alltogether?
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The numbers broadcast was supposed to unleash Nova 6 on America which would kill most Americans, then since we've never seen anything but the immediate effect of Nova 6, it possibly reanimates the corpses. After all, the Zombies in the Zombies mode and the people killed by N6 in game look similar.