An experimental drug went wrong. Originally made like any other drug, either get you high or calm and collective but this drug turned people into monsters. It started with people walking around aimlessly, then they would throw themselves in aggressive manner at people and objects, they attack and commit acts of cannibalism. It got worse as the people mutated and became the monsters that drug made them become. Zombies. You were running from a hoard of them, the city is in mayhem and just as you thought your life was done for once you tripped, you hear someone yelling

Hey!! Fuck-faces!! Over here!!

You look and see a man wearing a bulletproof vest and have strong gear on him and he shoots down the zombies that were in pursuit of you. Once they dropped dead, he runs over to you and helps you up

Are you all right? You hurt anywhere?