It is out of the lore and experience of the ancients and of all those who have studied the powers of the Un-Dead. When they become such, there comes with the change the curse of immortality; they cannot die, but must go on age after age adding new victims and multiplying the evils of the world; for all that die from the preying of the Un-Dead become themselves Un-Dead, and prey on their kind. And so the circle goes on ever widening, like as the ripples from a stone thrown in the water.
– Bram Stoker
Dracula, Chapter 16. Prof. Abraham Van Helsing, an expert on vampires, describes the undead. As well as being immortal, they prey on others to sustain themselves and when they bite and kill their prey they turn them into vampires.