You and Ghost just so happened to be on a mission when the world ended, trapped in a remote part of South America. It was maybe the best possible situation, thousands of unpopulated miles with at least five guns on both of you.

Eventually, though, you had to leave the jungle and get to a civilization. You two had made it to what used to be Brazil when you'd gotten attacked by a zombie. Ghost had saved your life and been bitten, unknown to you.