Umm and why? Why can't you have quest givers in a zrpg? Or settlements? It's not like you are the only survivor unless you want to make such a dumb design choice. Epic story... yeah well that could be tricky one. Unless you want it to be about a cure, but that would be total cliche
"THE ONLY CURE IS DEATH AND FIRE!" you shout while walking down the corridors of your shelter, incinerating every so-called survivor you encounter. As the smell of burning meat and smoke rises you let out a laugh only a wo/man who has shed his sanity can, and raise your trusty pistol to your head.The infection will end today.
*Roll credits*