Possession

Anything can become possessed; plants, animals, humans, and even inanimate objects.

Possession is what happens when a more powerful entity, usually a ghost or demon, invades a host body by force. Sometimes this can be relatively immediate, sometimes it happens over the course of weeks or months, or even longer depending on the entity in question.

Ending a possession is difficult at best, death for the host at worst. Depending on the entity their removal could cause all the injuries sustained by the host to rapidly appear and be lethal, and few possessing entities will willingly give up their host body. Often times religious ceremonies are needed, as well as restraint of the host which is easier said than done as the possessing entity will do everything possible to prevent such actions.

And even then, a host may seek out possession once more or at least be more open to it again. So ending a possession is often not logical, while the destruction of the host body is; the entity can be trapped inside and destroyed along with the host body, and the host's suffering isn't prolonged beyond that point.