DOES POSSESSION EXIST?

The simple answer is yes, it does. It's more common than people realise and I see it regularly, once you know the signs it's easy to spot.
Religious organisations also recognise that possession exists, the Churches don't employ exorcists for the sake of it and in recent times more exorcists have been trained to deal with demand. The Catholic church admits that it has exorcists and always has, it documents and relates stories of possession and their subsequent cleansings, there are no secrets. They have even made TV shows to demonstrate the depth of the problem.
The best tip to spotting possession is to learn to read eyes. It's said that they are the windows to the soul and in part that statement is true. We have all seen pictures of criminals or terrorists in the newspapers and we see the expressions on their faces, we see the blackness in their eyes and we say 'how could anyone not see it? it is so obvious.' It really is that clear to see, you just have to bother to look. We all see people on TV who are full of arrogance, over-confident without the talent or ability to back up their boasting. Such people are not just simple minded they are instead possessed.
There are different grades of possession. A mild case would be someone whose personality only demonstrates the lower self. Such a person may have many vices, be obsessed with sex, smoke and drink too much, be over-confident, arrogant and over-bearing, they may have little respect for others and insist that they are always right.
Someone who has a spirit or entity within their aura may have subtle personality changes and ideas that don't seem to be their own. They may withdraw from people and act out of character. They may also be more tired than usual as the attached entity draws energy from them, its aim is to get into the physical body so until then it keeps a low profile and hides in the energy of its host so as to remain undiscovered. (A note to the healers amongst you - you should always cleanse a client before giving healing energy or you will be merely feeding the entity and making it stronger.)
When the entity gets into a persons chakras more profound personality changes will take place. You may notice a definite change in behaviour with the person concerned changing almost from one minute to the next. They will be argumentative, disrespectful, full of self importance, swear, be aggressive or passive aggressive. Also they will become manipulative, crafty and lie to get their own way. They can smoke more, drink more, perhaps take drugs and be more sexually active and aggressive during sex. They are self-interested and hard faced, their character changes although the basic self remains and you will recognise them, you will feel uncomfortable around them and would rather not be with them.
A demonic possession will be all of the above incorporating childish behaviour and pettiness coupled with  violence both physical and mental. They will display loss of control, temper and anger when obstacles are put in their way and all of the above will be exaggerated. They are more ambitious, want fame and have less inhibitions. They will do everything that is bad for their health, the aim of the demon is to destroy the spirit, the light that is carried by it and to feed from the negative energy the possession creates until the ultimate destruction of the possessed person. The demon will lthen move on to the next host.
The earlier a possession is recognised and dealt with the better the chances are for a full recovery.
My tip is to look people in the eyes, get the measure of them by what you see and sense. Don't worry too much about people who can't look you in the eyes in return, some people are shy, sensitive or private, they have nothing to hide but don't have the confidence to meet you head on. A demon or bad spirit will more often look right at you to intimidate you so take the opportunity to have a good look.
Usually people who end up possessed are weak minded in some way, but all possessions can be healed by someone who knows what they are doing.
Possession is not to be confused with mental illness conditions. There is an overlap at times of the two but mental conditions also exist and have their own identity, each case must be examined individually.
To avoid possession yourself, be good, take of yourself and those you love. Don't over-indulge. Take exercise, laugh a lot, have a good work-life balance, see friends and don't brood over things that go wrong. Life throws many challenges our way and most people don't have time to worry about spiritual matters as well as physical ones but it pays to be aware of your inner self and its needs, your moods and when you're not feeling yourself. Most people can shake off a potential possession by being balanced and laughter and happiness always shake the grip of the negative forces right off.