coyote6
Adventurer
Bird flu spreads orders of magnitude faster than ZA. ZA would spread pretty slowly, especially once it was identified.
That assumes the zombie-causing agency is only passed by contact with the zombies. If anybody that dies for any reason turns into a zombie, and zombie bites just kill you & zombify you faster, then the apocalypse gets a lot more apocalyptic. How many people die every day? They're all zombies. And they're probably zombies before the people at the morgues, mortuaries, cemeteries, crime labs, etc. are aware that they should be ready to re-kill corpses.
I'm pretty sure at least some zombie stories have something like that as a basis for the zombification.
Then there's zombies a la King's Cellular or the final episodes of Dollhouse (neither of which actually zombies, but serve much the same function), where people are "converted" simultaneously, and over broad swathes of territory, if not world-wide.