Turns out the whole zombie shebang is some unexplained phenomenon to evolve humanity into one perfect being, free from life and death, free from the burdens and the pain of the individual or whatever(an old hat since Evangelion). Worst part in my opinion is that the high-school girl Hideo meets in the suicide forest is a major catalyst for this event In Japan. She gets absorbed into your generic, organic humanoid mass of all zombies and saves Hideo at the end from death. A couple of other people survive the doomsday and continue to piss their life away in the post-apocalyptic world. Hideo wanders around as the last survivor of Tokio and the others chill on some island. And that's your lot. No closure on what the fuck was going with all that supernatural business or what happened to humanity as a whole or anything really. Very unsatisfying ending.

More or less, like with wasps and bees etc, when the Queen dies or is killed, the entire Hive dies. We saw it happen in Italy, and from what I can see Hideo killed the High school girl in that shot near the end. You can easily assume that the same event with the Queens being killed happened at every point all over the world, in the end the heroes always win. I don't think the manga was intended to follow the usual tropes that most manga do, if it was any other manga you'd usually find out the origin of the Zombies nearing the climax of the story. However I think the ending somewhat suits the story that the manga tells. Viruses' origins are usually unknown, spread incredibly fast, and even the world's strongest and smartest forces are powerless agaisn't. To explain the Zombie origin and story short, It was a virus of Alien origin intended to clear the earth while also creating immortal beings. Those who were somewhat immune to the virus was a destined issue, however they were also the perfect candidates for the hierarchy of the Hive. The ending wasn't satisfactory, but it still suited.