Demonic Possession

Demonic possession happens when Satan or another demon forcefully inhabits a human body. The oldest known references to demonic possession come from the ancient Sumerians, who believed that all diseases, of both the mind and the body, were caused by "sickness demons" known as gidim. Thankfully, the priests (known as ashipu, or sorcerers) had exorcisms of their own to banish these spirits.

Many cultures and most major religions have some concept of demonic possession, but it varies considerably. The Roma people believe that demons can also possess animals, the dead, plants, or inanimate objects. Symptoms of demonic possession include:
 * Erased memories
 * Convulsions and fits
 * Speaking in tongues
 * Superhuman strength
 * Access to hidden knowledge (previously unknown)
 * Drastic changes in tone of voice and facial expressions
 * Unexplained injuries (scratches or bite marks)
 * Piercing eyes (said in some cases to turn totally black)
 * Major changes in personality
 * Uncharacteristic aggression and antisocial behaviors
 * A disregard for modesty
 * An aversion to religious icons or sacred objects
 * Self-mutilation
 * Foul language
 * May exhibit supernatural powers (i.e. precognition, retrocognition, mind-reading, levitation, etc.)
 * Scratches or welts in the form of writing or symbols
 * Multiple voices emanating from one person
 * Compulsive sinning
 * Mental and emotional problems
 * Unnatural sexual desires

To make a long story short (too late), there are innumerable physical and mental signs of demonic possession, and demonic possession means trouble.