In Scientology, the concept of the thetan is similar to the concept of self, or the spirit or soul. A body thetan or a BT is a disincarnate thetan who is "stuck" in, on or near a human body, and all human bodies are said to be infested by these disembodied thetans, or clusters of them. Scientologists believe body thetans came about approximately 75 million years ago through a catastrophe brought on by a galactic dictator named Xenu, as described by L. Ron Hubbard in a confidential auditing (counseling level in Scientology) called OT III.