From Dan of Population Go:

One day a virus appeared on Earth, this mysterious virus kills every infected human over the age of 13. At the same time Vampires emerged from the bowels of the world and enslaved mankind. Enter Hyakuya Yuuichirou, he and the rest of the children from his orphanage are treated as livestock by the vampires. Even in captivity, Yuuichirou dreams big. He dreams of killing vampires. He dreams of killing all of them.