Demonic Possession.  It's extremely rare that you will ever hear of a full-fledged demonic possession. But it is something that all ghost hunters need to be aware of & briefed on. Full possession requires an "invitation" in order to gain a foothold. However, there are activities that can be construed as an invitation. i.e. Ouija boards, séances, etc. A person with heavy addictions, depression or someone who is desperately lonely are more apt to be targets. When any type of invitation is construed, whether implied or direct, it may very well be acted upon by such evil entities. Even with all that, it's still rare that evil will triumph successfully, but it is possible in some cases. Ghost hunters are somewhat susceptible because of the nature of their work.