Cool article, thanks for the link.
Personally I think a mutated virus is the most like to result in a zombie apocalypse, if not the obliteration of the human race. Look at the recent swine flu virus pandemic, ok it wasn't as deadly as say the pandemic of the 1920s, which killed up to 100 million people (yes, you read that right see here:
1918 flu pandemic - Wikipedia, the free encyclopedia), but it *could* have been a lot worse than predicted.
Imagine an AIDS virus mutating with a flu virus for example. You then then have an airborne AIDS virus. Imagine if that now mutated into a "zombie" virus.
Or how about this, imagine the rabies virus mutating into an airborne, flu-like virus. That's pretty close to a zombie virus, and it's well within the realms of possibility.
Good job I'm ready for a zombie apocalypse when it happens!