What's the origin of the concept of undeath?

[Valandris]

We all know what undeath is, what the undead (universal, not limited to WoW) are. We've seen variations of this concept and the reaction we would feel towards such beings (i.e. fear, disgust, horror) throughout history and popular culture in a global sense. The question I pose is:

Where did it begin, and what caused the idea to be prevalent amongst the entire human race, across various languages?

Don't try to answer with another question like "Why do we all know to eat, to sleep, or to reproduce?" That's an instinct shared by all similar animals. I'm concerned about a concept that, to me would be unique to humanity, provided we can't communicate to animals to understand exactly what they know and think.

[NewfieDave]

I don't know exactly how far back it goes, but I watched a documentary about vampire lore once that had some interesting facts. Gas builds up inside a dead body, and when grave robbers would unearth a corpse, they would sometimes hear sounds like the body was exhaling, or even screaming. The "wails of the dead" so to speak. These sounds were actually just the gas being released, but in an age of lesser science it scared people into thinking the corpse was somehow alive again. There are many stories like this that started and spread throughout folklore. The concept of undeath goes back a long, long time.

[Valandris]

I actually watched that before myself. Apparently they believed the bloating was from drinking blood, similar to how a tick looks after satiating itself off of the host, plus there had been some mysterious deaths in that area during the time, so I reckon superstition ran wild.