To Save A Dead World

Fifty-three years ago, an asteroid fell to earth, destroying several countries. A strange chemical from this asteroid seeped into the corpses and turned them into...well, zombies. Anyone bitten by one of the zombies turns like them. Except for those with the immunity. An accidental genetic passed on through generations after a factory where a chemical that was supposed to create immortality was blew up. If you would like a better explanation of this, since I am bad at summaries, read my story!