Incomplete and buggy as it is, DayZ (the terrifying zombie-survival modification for Arma 2: Combined Operations) is a superb and compelling piece of work, with more players trying to play than can comfortably be accommodated on its large list of hosted servers.
Now, you put a foot wrong and you can get pulled down by zombies in less time than it takes to tell. When one comes after you, it may bring more. Actually taking a shot at it will almost always bring more.
But it isn’t the zombies that you have to worry about.