OK SO. He got bit, passed out, then woke up with 10 year long Amnesia. I initially thought the multi armed monster he saw was real but Zessman raises a good point. Perhaps thats how the zombies see humans. Then again, it wouldn't make sense to rush and bite what you perceive to be disgusting monsters.
If what he saw WAS real, then I believe the zombies are able to fuse together. Maybe that's why they're all heading south? To fuse into one mega flesh beast... LIKE AKIRA!