In his book Entity Possession, Samuel Sagan, M.D. presents an inventory of non-physical forms that can attach themselves to humans. They include: Fragments-- fractured characters that break off from the astral body of the deceased; may focus on a particular negative or obsessive trait. Perverse Energies-- An etheric substance that has infiltrated the human field, sometimes perceived as a faint or blurry presence. Elementals & Natures Spirits-- Small beings associated with wind, fire, earth, water and plant life; can sometimes contribute to health & mental disorders. Rakasas-- described in Sanskrit as intense entities that can live for hundreds of years; their foul odor is said to render humans insane.