Linked below is an insightful, if not terrifying, article from one of my favourite sites:

This article describe 5 zombie-related scenarios (as presented in contemporary film) that are, theoretically possible. While raising your favourite zombie flicks to a level of reality that you neither desired or anticipated, this article provides a few laughs and a handy ZOMBABILITY scale for likely occurence.
5 Scientific Reasons a Zombie Apocalypse COULD ACTUALLY HAPPEN.