The joint Europe and American governments created a biological weapon to use to attack the opposing sides in Iraq and other such warzones, but they had no idea that it would evolve and become airborne:
firstly it only targeted the recently deceased, but as the virus entered the dead's bodies, it took over the muscles, 'causing the corpse to have basic human instincts: eating and walking. With only the sense of touch and hearing.
Soon after a few of the walking corpses were hidden from the public eye, more rose up and it only took one bite catch the virus; it was impossible to catch it anyother way except from being bitten or being dead.
You can have a max of 2 characters, and as many weapons as you like. Your characters can have a crazy background, or a normal one.
Gore, swearing and any other thing that don't violate the Protagonize rules are allowed :D Romance is cool with me, as long as you have the other authors permission and don't make it revolve around their relationship.
Also, ask before you kill off characters: if it's your own you can either pass it on to another author or just kill 'em.