Wraith is alien race from another galaxy (from the TV series Stargate: Atlantis). This hybrid beetle called Iratus and people.
According to the first theory Iratus preyed on humans (sucking blood from them) and thus absorb the human DNA into its own.
Wraith feed in a similar manner: they have on their hands a "feeding slot" which they suck the vitality of the people (people are with them closest relatives, so their vitality best meet dietary needs of Wraith). As a result, the Wraith are virtually immortal - they do not age, but they can die from too many wounds.