It was a curse that the boss in Zul'Gurub would give to you, and would be passed to people within a certain distance of anyone infected with it. The bug would be that it was possible to keep this curse outside of the instance...making it easy to just walk around a populated area causing mass murder.

_________________
The body does not live. The soul lives. The body is simply animated by the soul. You have never seen anyone in your life (with your physical eyes). You have only seen their body, or rather, an electrical image of their body. The human body is basically a high end animalistic machine designed by Nature for the expression of the Spirit by way of the Soul.