Once someone's suspicions starts to rise, though, everyone freaks out. And attacks each other. Anyone could be the infiltrator! Little do they know that the real infiltrator is hanging back, waiting to finish off the rest of the group members who haven't already offed each other.
Desperate times, where survival is bleak and people can't be trusted anymore, really makes for high tension even amongst groups of people who've banded together. DayZ, and this video in particular, are a great example of just how wrong things can go. It often seems that paranoia and selfishness are more a cause for death than zombies in this Arma II mod.