What are the rules for Zombivors?
The official rules for Zombivors are now available here.

Quick recap:


  Death and zombie infection can’t turn a real hero into a monster .
  Your favorite Survivors can make it back to the game  as Zombie heroes
  called Zombivors! When your Survivor is  eliminated by Zombies, you
  can turn him into a Zombivor:  flip his Identity Card and savor a
  bloody revenge . The Zombivors’ status is indicated with on the
  Survivors’  Identity Card . All Survivor rules apply to them . The
  killer feature about Zombivors is their incredible endurance: a
  Zombivor is only eliminated by five Wounded  Cards in his Inventory
  (instead of two for a standard  Survivor) 
  
  Zombivors are great tanks . There’s a serious drawback, though
  (besides the foul breath): at Yellow  Level, Zombivors don’t gain the
  +1 Action Skill of  standard Survivors


It goes on to explain that you can either play with them starting as zombies, or play with them starting as normal but coming back as zombies if they are defeated (which obviously is a big advantage).