I wonder what happened before the flora and Fauna ran everybody out of the cities and towns. Why is the central character still there? What is the importance of the factory that is broken into? What was in the box? I would like you to make a plan of what happened before. Plan what the prequel story could have been and make some jottings to show your plan and initial ideas.
Then you could write some of the narrative of this prequel story. Think about how you will show character, setting and atmosphere.I will present some of your writing tomorrow on the website. Email to firstname.lastname@example.org