Demonic possession is a natural ability similar to the spell magic jar. The demon assumes control over the host body and thereafter determines the actions of a creature until such time as the demon abandond the body or is forced out through the spells exorcise or abjure. Most demons possess this ability.
Before the demon can succeed in possessing its object, the would-be host is entitled to a saving throw against paralysation. Success enables the host resist the possession. Normally, demons like to choose hosts possessing a low wisdom, which gives penalties to the saving throw if low enough. Rarely do demons choose robust or highly intelligent beings. For them, any body will do. This sometimes means that an individual lacking in wisdom or intelligence will begin acting out of character, which is often taken as a sign of possession (even when it is not).
As well, since a demon will also take possession of an unhealthy or dying body, which then quickly recuperates and becomes "well," some will often assume that anyone who recovers from a disease in this manner must therefore be possessed. It is common for fanatical sects, resistant to reason or empathy, to burn "witches" under the certainty that these are persons possessed by demons, though it is more common that these poor wretches are innocent.
A demonic possession will be revealed by the spell penetrate disguise, though it is often believed that some demons are able to evade this spell and make it appear that the host is not possessed.