There was unethical scientific experimentation involving a chimera virus engineered to cure diseases. Due to not being in the right hands,the virus morphed into something worse,worser than the human mind could've comprehended back then.

The world crumbles over the following few months. Mindless creatures known as zombies now roam around.

Izuku had entered a small band of 7 of survivors including him. He had learned their names were Katsuki, Mina, Kirishima, Denki, Uraraka and Tsu.