You don't know where they came from, but all you know is that you need to fight back as they would eat you. They won't stop till they die and that would be hard since there brain is already dead. You need to protect the outpost from zombies attacking. Each kill would earn you money and you can use the money to buy some upgrade. You can either buy new weapon or upgrade your stat. Either way, you would need them as the game keeps on getting more difficult. Don't expect things to get easier as the zombies would be tougher as you make some progress.