From the previews it looked like what it should have been is the story of a group of people randomly trapped in an apartment building together when the government decided that, though they were not all infected, the safest thing to do would be to let them all die, and not just let them die, but actively take a role in killing them by preventing them from escaping. At which point the people would escape anyway and begin a covert war against the shady agency that tried to kill them, eventually destroying it by harnessing the infection as a weapon against said agency. (I picture this as involving rats, or darts, or both.)
And then it would turn out that the agency's policy of human sacrifice was actually the only thing keeping the constant zombie menace in check. Infections pop up all the time and all over the place and without someone there killing everything that moved the infection simply couldn't be contained, it was too widespread. So civilization would collapse and there would be a question of whether or not the means (preventing the murder of innocent people by the government) were justified if the resulting end (a world overrun by zombies) was pretty damn bad.
I get the impression that that wasn't what the movie Quarantine was about, even though the previews screamed out for that to be the case.
Anyway, if someone is actively working against the primary thing holding zombies in check, that changes everything.