When zombies roam the Earth, everything will change around you.  You will sudenly found yourself clinging for your life most of the time.  Zombies will always come rushing to kill and eat you.  It will be non-stop and you need to kill in order to live.   The game is non-stop as zombies will always appear.  There won't be any level since you will need to defend yourself at all times.  Pick the weapons that are dropping from the sky to have a better chance.  Some weapons will give you a better chance of surviving than others.  The idea here is to last as long as you can.