You've been surviving in a post-apocalyptic world for a long enough time now that you know your way around. You know how to avoid the zombies, and how to fight them if need be, you know where to go for shelter and for food, medical supplies. All in all, you were doing well for yourself. Well, as well as anyone could in this kind of society The only problem is that you were recently injured and you're not sure how much longer you can walk on your leg with that wound, and you're too far from your shelter to walk. That's how you ended up where you were now, standing in front of a disheveled building you saw while travelling and preparing to knock, hopefully getting some help from whoever's on the other side.