The idea that demons or evil spirits or even spirits of the dead can live in your body and at times control it is popular.  Exorcisms in religion or rescue work in Spiritualism is what is done to hopefully get the entity/entities) out.

A possessed priest called Fr Surin wrote, "I cannot explain what happens within me, and how this spirit (the evil one) unites himself with me without depriving me either of my liberty or my consciousness.  He becomes, nonetheless, like another self; it is then as if I had two souls, one of which is deprived of the use of its bodily organs and remains afar off, watching the actions of the one which has taken possession of the body, while this other acts in the body as if it were its master." True or False Possession? by Jean Lhermitte.

The testimony of many alleged victims is that they feel they are the demon in some way.  Yet there are methods of exorcism that verbally abuse and rile anger against the demon or spirit.  These are clearly attacks on the victim.  The "spirit" invariably starts degrading and abusing the victim during the prayers.  Is the cure worse than the disease?  Nobody sane sees any love in an exorcist.

How can you be sure that the "victim" is not the one doing the possessing?  Or what if demon and victim imagine or assume they possess the other?

There are a growing number people who are led to believe they have demons residing in them.  They are reportedly possessed.  In Christianity, a demon is typically a fallen angel so it is smarter and more powerful than man.  After all Jim down the road cannot live in my body but a demon supposedly can.

It is asserted by some that demon possession is essentially a spiritual malady.  That makes it simple for mental illness is a multifactorial affair. It is very complicated.  Chemicals, food, trauma, your social setting, where you live, what you own, how you tend to feel and think and your religious and spiritual endeavours come together to produce an illness.  If insanity or schizophrenia or something else is mistaken for possession then that is very dangerous.  It is over-simplifying a mental illness at the very least.  Mental illness will become more dangerous if the person is told she is possessed for it will condition the illness to act as if she is.
The victim during an exorcism is often aware of what is going on. The process must be very distressing or her or him. Not only is there a creepy ceremony going on, but it is often being repeated and it builds up hope that the demon will go and that hope is usually dashed. It reinforces the belief that there really is a demon there. And the "demon" may be rampant and crazed during the exorcisms and start tormenting the victim with new ferocity. The victim is put through all that when there is absolutely no evidence that exorcism really benefits. The demon may go in its own time or start to hide in the victim. Exorcism is abuse. When the victim is a child this is heinous abuse.
Exorcists have no way of being sure that when they throw holy water on a person who screams that it burns if it is the demon or the person who is speaking. And the demon may be speaking but what if the real person is suffering too and cannot speak? The demon could leave temporarily or hide in the person to avoid the pain. A real demon would surely know that sprinkling holy water is part of the routine. It would see the bottle. It would hide. The reaction shows that the demon is faking if there is a demon. The person suffering, if any, is the victim. Holy water on the body of a person who is not the demon cannot effect the demon. The Wicked Witch of the West would not have been melted by a bucket of water had she being wearing waterproofs. The body of the possessed person does not belong to the demon so the demon cannot respond if the body is sprinkled with holy water. The body is like a shell.
Is possession a symptom of a mental illness or is mental illness a symptom of possession?  If a person can be possessed and its not their fault then how is the demon getting in or what drew its interest in possessing?  Does mental illness or a psychological disorder open a door?  If there is a door nobody knows what opens it and exactly when.  If mental illness is a symptom of possession then all mentally ill people should be considered for exorcism.  Religion has no answers for these questions which shows how irresponsible and unscientific it is.

The exorcist believes possession and mental illness can co-exist in the one person which makes anything that causes torment irresponsible. And the person may thrash about in agony and injure themselves.

A demon can use your mental illness to torment you at least psychologically.

If you have a mental problem and are possessed then if the demon got in and its your fault then the mental illness being still there or getting worse is your fault do for you did not have to let the demon in.
A person may claim to be afflicted by a demon and possessed. More often, it is those who know the person who make this claim about them. That makes the whole circus surrounding exorcism far worse. 
The doctrine that a demon cannot really take over your body is a smokescreen for the Church acts as if it does. If it were true that the demon merely affects the body but does not own it it would not react to the holy water. If it does then it is faking which raises the question of why it wants people to think the Catholic Church really has power over evil spirits? The victim if she has any intelligence will realise that and that could be dangerous on the psychological level.
The Church never says that the person or "victim" has become a demon! Who says that some spirit has to come from elsewhere to possess you? What if you become a demon and, as it were, possess yourself? Why do people think you have to die and go to Hell before you can become a demon? The demon is not going to be reliable so you cannot believe the information it gives about itself.
If you say a person has a demon, you might say it is possible that they are witches and lying that they are possessed. In that case, rather than have a demon they are the demon! It would be the ultimate in hate speech to suggest somebody is a demon or a servant of Satan the greatest liar and murderer imaginable (according to Jesus Christ). If you believed that about a person you could be regarded as guilty but insane if you killed them. It would be a crime of passion. When priests cannot really know much about the alleged entity, they are as good as saying the entity may be the person herself.
Once you say a person has a demon you by default show that you think they might be faking and might be the demon. You only assume it is likely that they have a demon so part of you has to be agnostic. In that case, exorcism would be no good. Religious people have no right to say, "We are 100% sure that the person is not evil but possessed." It is arrogance for it is impossible to know it 100%. And it is a lie.
Belief in possession by evil spirits of dead people or demons or deluded spirits of dead people who now think they are demons is dangerous. It opens the door to accusing dead people of what they have not done. Consider how an innocent old man who died in his chair was blamed for the Enfield Haunting in London. That case is a hoax though it was better assessed than Catholic accounts of demon possession.
Demonic possession is linked to accusing people of cursing the victim or of the victim inviting the demon in. If a demon enters by itself it will blame somebody to cause trouble. Being cursed and then possessed cannot prove the curse caused the possession. If B follows A that does not mean B was caused by A. It is easy to waste time exorcising demons when you see no effect and when you tell yourself that your Jesus gets it out but somebody is using magic to put it back in.
Psychotic people often report visions and terrors of a religious nature. They may see Satan appearing to them. Or Jesus may be threatening them with eternal torment. For the priest trying to see if the person is possessed, the information about the visions will prove very important. He will suspect that if they are down to the illness they are also caused by demons. Or he will think they are all down to demons. But that leads to the risk of misdiagnosing. And if the visions show signs of being down to mental illness you can assume that Satan is making sure that it looks as if it is the cause. He would. There is no way to diagnose at all. The diagnosis is just a guess and isn't fit to be described even as an opinion.
It stands to reason that if possessions happen there have to be possessions that are unknown and undetectable.  If demons control the body, they can pretend to be the person. They may commit murders. If demons control the person and not just the body, it follows that many murderers could be innocent. The Devil made them do it. The concept of demonic possession then is extremely important for it has huge implications if it really happens. The trouble is that if demonic possession is real, then different religions have different opinions about it. The door then is opened for people to think that demons are murdering and that murderers are innocent.

Roman Catholicism is a dangerous fanatical religion for encouraging the notion that there is such a thing as demonic possession. What if a new Hitler came and nations listened to the Church and refused to wage war on him because the church starts to say, "He needs a good exorcism. It is wrong to fight him for he is possessed"? The faith is intrinsically fanatical. The responsibility for this is down to Jesus.

The gospels indicate that the demons are so desperate to be in bodies that they would happily inhabit pigs even for a few minutes. Jesus put demons out of a man and they pleaded to be sent into pigs which then went mad and drowned themselves. The Catholic Church claims that it can cast out demons with prayer through priests conducting exorcisms. Demons would act as if the exorcism worked so that they could stay in the person. We have the gospels as our authority for being suspicious. And especially when Catholicism does not really do exorcisms. It only imagines it does. The priests keep praying until the person seems to have recovered and that takes ages - decades at times. That is like praying for a sick person until they feel better and thinking you can cast out sickness. Jesus's exorcisms were allegedly instant. He would say that the Church is counterfeiting the power to cast out demons - and he would see it as a demonic faith and/or a delusional one.
The best person to judge if an exorcism worked or not is not the exorcist or anybody else. It is the victim. And yet the one person who should know may not know.  How can they when the demon is so much smarter than they? Why do those who want us to believe in exorcism and have high regard for it neglect to provide us with the victim's witness? Why do they not even seek such witness?
Some people are believed to be possessed by evil spirits or demons. Some people fake their possession to gain attention. Some manifestations mistaken for diabolic possession are actually psychological or physiological conditions. Some possible causes for these pseudo-possessions are: migraines, bipolar disorder, Tourette's syndrome, somatization disorder, multiple personality disorder. If possession is not genuine, then it is the acceptance by the Christians of the myth of possession that produces the suffering of those who think they are possessed.
The Catholic Church claims to have the authority to cast out demons from people who are tormented by them. It has a rite of exorcism for that purpose. We read of how Jesus supposedly cast out demons in the gospels. Unlike the Catholic Church, the gospels say Jesus got instant results and just ordered the demon out instead of needing ceremonies and prayers. The differences are striking and you get the impression that the Devil is only making it look like the Catholic Church can move demons out of a person.
If the demon is gone, it is possible that the Church has made the person think he or she is still possessed!
Conspiracy theories wreak havoc today. Belief in a Devil and demons who intervene in world affairs only leads to a nasty new batch of conspiracy theories. Eg, maybe Barack Obama is dead and a clone controlled by demons is now the President of the United States? And there is no hope of answering that. With conspiracy theories, there may be some hope of making the believer see sense but only as long as the believer makes no supernatural assumptions. There should be ways of showing that it is implausible that the Freemasons control all the world's finances. But if they have magical powers you cannot know that it is implausible. See the point? Belief in demons and the devil and exorcisms is essentially evil.
The Chief Exorcist of the Diocese of Rome, Father Gabriele Amorth, claims to have performed in excess of 70,000 exorcisms. Yet he says he surmises that less than 100 of these were cases of real demonic possession. This is a man who misled thousands of people. He treated them as if they had demons when they had not. What would we think of a doctor who let people think they had cancer when they had not?
People might say that as long as the exorcism helps the people it doesn't matter if there were really demons there or not. That is saying that human happiness matters more than truth. Should we start then pretending and saying that nuclear weapons are a myth for that helps people to feel happier. Those people are still getting the wrong treatment. They would be better off with the right treatment no matter what they think or feel. They are still being cheated.
Exorcism may seem to help a few. But most people have to endure it time and time again. Is the torture worth the release? They have to decide. Exorcists claim that it sometimes takes decades to get a demon out of a person. That to me shows the rite of exorcism is not very effective. The victim is tormented for years by false hope and a scary ritual that reinforces their belief in the power of the demons. The exorcists care more about expressing and promoting Catholic doctrine by their practices than the victim.
Amorth is performing exorcisms which can encourage people who are suffering mentally to suffer far more because they have started to think they are possessed because of the wonder tales of nutters like him. The shocking figure of 70,000 + certainly implies that this is what is happening.

Why would there be so many demons possessing people in Christ's day when Amorth only has met 100? Why didn't the demons keep out of Jesus' way? There is something contrived about it all. They either were on the same side as him meaning he was evil or the gospels were lying. He put the demons in them perhaps to get attention by pretending to cast them out when they merely intended to leave anyway. Or the demons only wanted to possess people in order to promote Christ.

Religion judges possessed people as somehow having invited demons in.  What about possessed children or troubled young people?  Possession victims cannot give informed consent so how come they end up possessed? God is more to blame even than Satan.  The invitation thing means the Church can blame the victim if a demon just won't shift.  They can argue that the person may not really want rid of the demon at all which is why the prayers seem unable to eject the demon.  That would be a very cruel insinuation and it means the Church is taking advantage of the fact that many sick people do seem to get better anyway.  When a person with severe emotional problems stops claiming to be possessed the Church exploits that person as a triumph if it has performed an exorcism on her or him. 

If exorcisms are so good why are they invariably conducted in the person's presence?  They should work if the exorcists are on the other side of the world.  The religions of exorcism are just brainwashing the victim.  It is further victimisation of a vulnerable person.

Possession is a crazy belief and strikes at the heart of what a person is.  You are you and if there is a demon it is not real.  It is those promoting the belief that do the harm so we should maybe give demons a break?

We find that exorcisms imply or claim that the victim may be responsible for inviting a demon in or for being the demon in the sense of an artificial personality. The person may have supernatural powers to make the demon herself or himself to get attention.