The story is reminiscent of Portal's ambient narrative; writing scrawled on walls hints at other groups of survivors and back story is tossed around in short conversations between the characters. Things get conspiratorial when it is revealed that the military is getting a little trigger happy with civilians; and that those who aren't infected may be 'carriers' of the disease. Indeed it seems more than likely that the final chapter's glorious helicopter escape is simply the prelude to either quarantine or death. You're not told, but it doesn't matter because the story is over. In a game, what use is plot after the player stops playing?