If this virus is spread via saliva, then a world wide outbreak could be very easy. You could be contagious before ever showing "zombie" traits in your behavior.
And never underestimate the power of sheer zombie numbers. The Germans basically could never defeat the Russians because the Russians had more bodies than the Germans had bullets.
And there has to be people out there. You could have nuclear subs or weather station outposts that could be still existing. The key is trying to communicate with other people. How do the remaining people find each other?
And what was whispered?
My guesses are...
The CDC started this entire mess.
The zombies will wear down and die. You just need to outlast them.
That is my other questions for World Wide Z readers, do zombies die? Do they wear down? They need food for energy, right? A staple of science is all living things need energy, and outside of the sun, nothing creates its own energy from basically nothing. So zombies will wear down and die unless they start eating each other.