I'm not sure what's happened but I was on the top floor of a 3 story building prone, I finally lost a zombie that was chasing me. I stood up, into crouch mode and instantly pulled every zombie on the ground. I had no weapon or bandages left so I had to die. I can't really get around anywhere without prone now. They hear me easily around buildings, fences, and 3 stories up from the ground. I'm not one to bitch about the game being too hard, I like a challenge but this is a bit much.