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I've Read that they can only do it if you invite them intentionaly or unintentionaly.

First of all true spirit/demons/ghost/whatever possession are rare most can be explained through illness or some sort of psychological or social causes. Hysteria meant to be the most common reason for so called Possessions, this is what some believe happened during the Salem Witchtrails.

But there are thousand of cases where this can't be explained away. Pretty much ever religious has some sort of spirit possess going on it in. When religious is involved it is normally an evil spirit of demon that is doing the possession, but in some cases it is believed that the person is being possessed by a god or holy spirit.

People that are possessed normally have become easy targets to possess by 1. opening themselfs to the spirits, 2. becoming ill and there for lowering defences, 3. they have something in common with the spirit or 4. as Dante said the spirit is stronger that the person it wishes to possess and forces it's way into them or to put it in a not so nice way the person has a weak mind.
For some reason Mediums seem easier to possess that normal people.

A lot of recent possess seem to have something like an ouija board involved, someone was using it and then got possessed or something a lot those lines. Other times it can be an everyday item. I heard about one case where a women was given an old backpack which she used a lot. The artical said that a bad spirit had possessed the bag and as the women wore it more and more it slowly began to possess her.

...slow that was a really long post...

not a huge believer in possession and feel like above most is just people releasing bottled up emotions. there are a small amount that are genuine and they usually are done to people that are weak, very psychic, or just a really strong spirit. items usually have placed memories rather than being possessed. you can have an item that is 'haunted' and what this means is that a spirit is drawn back to it. they are not necessarily trapped inside it, but it is like a beacon that calls them back to the physcial world. makes sense i hope.