Wouldn't it be cool if there was a second Undead Nightmare. Here's my story for how it would start.
Everything up to the final mission of Arthur's story would be the same until the moment when Arthur goes to confront Micha and they get into a draw. John walks out of the woods like normal but this time somethings off about him. He walks slowly groaning and has pale greenish skin. They gang all looks toward him and Bill asks, "John is that you". Marston walks closer but no response. Then light shines between the trees to reveal johns face which is ripped up and has blood all around the mouth. Micha sees this then says "What the hell" as he turns his gun toward John and shoots him 7 times in the chest. Which makes John fall to the ground. Arthur yells "NO!" then John quickly stands up and jumps on top of Micha devouring him. They gang watching in shock then hoards of more zombie flood from the woods. Dutch yells for everyone to get on their horses then everybody including Arthur rides as fast as they can into the woods separating on the way. While Arthur is riding he hits his head on branch which knocks him out. He wakes up in a doctors office strapped down to chair. He manages to break free then the doctor walks in. Arthur grabs him asking why is he here. Doc replies quickly saying he was testing some medicine on him to see if it prevented the undead virus. Arthur asks what that even is that then lets go of the doctor. Doc explains the virus turns people hostile and brain dead and that its transferred by bite. Arthur says that he was never bitten. The doctor says but you were so pale which Arthurs responds with, "I have tuberculosis". Then Doc looks at him and says "well you don't anymore".
