After the nukes had dropped the earths population decreased more than 75%. The remaining humans hid out in bunkers for years until it was 'safe' to head back into the natural world.

Izuku was part of the survivors. Or- was. He wasn't doing so good.

One of the ghouls that 'survived' after the blast had gotten him when he was distracted. He was barely managing to keep it from fully killing him, flat on his back and fighting to keep the ghoul off.