Their an infectious virus that caused everyone to become zombies except people with blood type A which you were as you were constantly on the run finding new shelter's and avoided being killed as one day a zombie managed to tackle you down and was about to kill you saw its head blown off

"I'm part of the Z force do not worry you'll be staying with me until we find a way back to HQ"

A fully armoured man kicked the dead body off you as he checked your body's state while speaking in a strict tone. He was obviously separated from his team meaning you were stuck with him

(The Z force is a group of survivors trained to search for more survivors and kill zombies)