Diseases can exist for centuries before becoming plagues under the right conditions-- the Black Death and AIDs are examples. There may have been individual instances or small outbreaks throughout History before the Zombie Apocalypse finally struck (although some government lab may have been trying to weaponize it or something).
Well, that goes with the territory! With the exception of the horror/comedy Return of the Living Dead II (where the zombie problem is solved, and the heroes survive), I cannot recall another where the future was anything other than the death toll rising or permanent dread for the survivors.
Does World War Z count? Granted, it's a book not a film.
I was thinking the same thing. They came out of it pretty well.