Chris Friend
In much of the folklore concerning vampires and the undead there is a considerable amount of lore on holding vigil over the dead. This was usually done to ensure that an animal usually a cat would not leap over the body causing it to become a vampire. Found universally, the belief held that the corpse could absorb enough of the life force from the animate creature and be given a kind of un-life. Even though cats were the usual culprits, other guilty animals such as dogs, bats, birds (usually a black rooster) as well as people could disturb and reanimate the corpse enough to make it undead. Besides animals, lit candles and even the wind were thought to cause the dead to reanimate.
So prevalent was this belief that in the Middle Ages monks were often assigned to keep vigil over the corpses of wealthy and high ranking individuals.