"After aliens constructed an elevator from Darwin, Australia into space, humanity established orbital colonies along the elevator's cord. Years later, those outside of the machines protective aura were wiped out by a mysterious plague. When the elevators virus shield begins to break down, a scavenger and a scientist must unravel the mystery of the failing alien technology to save what's left of the world."
That's it - go buy the book - I doubt there's been a finer novel about an elevator in the past 40 years. [I would have to read it to push that back past Roald Dahl's Charlie and the great glass elevator.]