Doomsday: I'm Actually the Zombie Mother

“Zombies?”

“Heh. That's just an excuse made by people whose willpower isn't strong enough!”

“We're helping all of humanity evolve. Those whose willpower isn't strong enough and can't persevere will become mindless walking corpses. Those who make it through will evolve, with improvements to every bodily function: their intellect, speed, and strength. They can even keep their cells active and gain immortality!”

We call such people... the New Humans.