The Rituale Romanum (Roman Ritual) a book containing the official Catholic exorcism ritual in Latin.
So, should only a priest perform exorcism?
A priest or other legitimate minister of the Church chosen to exorcize a demon should be endowed with piety, prudence and integrity of life. He should rely not on his own but on divine power to perform the undertaking humbly, firmly and out of charity, and forget all human affairs. Moreover, he should be of mature years, and revered not just for his office but for his moral qualities. Exorcism cannot be performed by your own will alone since demonic entities can be much more powerful then any regular entity.
Symptoms of demonic possession:
In the first place he should not be fast to believe that someone is posessed by a demon, but note whether the person has signs by which person possessed differs from those who suffer from a disease. Signs of posession are these: Speaking unknown language in many words or understanding when foreign language is spoken by someone else; Ability to divulge future and hidden events;Powers that are beyond person's age or natural condition; and other such things which together build up evidence. Although we practice some of these abilities ourselves, ones you dabbled with the occult as much as some of us did you'd know true signs.
Although human spirits can possess people(for example during certain types of mediumship and seances), this exorcism is for demonic possessions and seems to be the most powerful form of exorcisms. There are many forms of exorcisms from different religions but most people find catholic ones more powerful due it's ancient existence. Some before catholic was even born.
. I agree as well that some people who they thought were possessed was just the result of an illness. Sometimes I get extremely sad for those people but as well I am terrified of demons. I can stand ghosts but my 2 main fears are height and demons. But I still keep digging into them though. I sense a new blog on possessions coming from Mark 
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