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.