Black bullet is great. You can totally get lost in it. Good story and great characters. Basically, some parasite started infecting people, and once the corruption rate got over 50% they would turn into gremera (i spelt that so wrong) and basically be giant versions of what infected them. They cant be killed normally cuz they're super regenerative and can only be killed by a black material called varanium. If The infection is inside a child when its born, they become a cursed child, granting them supernatural abilities. This also causes them to all be girls. They also happen to be the only thing stopping the gremera (again, sorry its spelled wrong) from killing everyone.