Possession is when a spirit of some form is believed to have taken up resicence in a human body. Modern science states that possession is a form of altered mental state and that spirits are a superstition, whereas many religions and cultures believe in various forms of spirits and their possession powers.
Possession may be malignant of beneficial, much depending on the cultural context of the person possessed. In western society, possession is believed to be the work of the Devil or a demon. On Haiti, and other places where the voodoo faith is strong, people will willingly offer their bodies as vehicles (horses) for the Loa to ride.