How is it that a zombie that can't keep a hold of a young boy's ankle can rip a man's rib cage open in one second?

How is it that zombies are always loud and dumb and clumsy – except for when someone's not watching them and then they become super ninjas?

How is it that zombies are dead and obviously decaying out in the elements for months and months and yet are still about the same in appearance and functionality as they were at the beginning of the outbreak?

Conclusion: Yep, it's definitely fairy dust and mushrooms.

They actually explain that in detail in The Zombie Survival Guide – basically, it's all fairy dust and mushrooms.