Just add Kryptonite. Shortly after the zombie plague started, the earth was showered by a mysterious green rocks. The survivors quickly found that the rocks severely weakened and even repelled the zombies. The rocks became incredibly valuable for use around surviving settlements.

If you want to add superhuman qualities to the survivors, then pick up Armageddon or Angel. Witchcraft magic would also be very useful.
If evolution were true, I'd have a self-lubricating hand.
--Scolastic Films presents Intelligent Design
http://movies.lionhead.com/movie/1168