Thetans are minds/souls that are immortal, so we're all supposed to be "thetans" in bodies. A really screwed up thetan becomes very passive and sticks to other people's bodies and afflicts them. You don't learn about Body Thetans until OT III, when you learn about Xenu (who caused the "body thetans" to degenerate to their present state). Only about 5% or so of real believing Scientologists are OT III or higher. If you learn restricted knowledge prematurely you're supposed to die from it, but since in real life that doesn't happy you'll just get some BS explanation as to why you didn't die. But if you go looking into that stuff online or whatever after you are already in the church they won't let you advance through the OT levels anyway.