The potential host is entitled to a saving throw against magic to resist the possession; but it should be noted that demons very rarely choose the natural forms of robust or highly intelligent beings, as any body will do. It is more likely that a demon will take possession of a common creature, who may then begin to display an intelligence formerly unexpected of that individual; or a demon will take possession of an unhealthy or dying body, which will then quickly recuperate and become "well," though now under the control of a demon. The more difficult to achieve saving throw for persons in these states, with a low wisdom [placeholder]or will to resist, virtually guarantees the success of the demon to possess the creature. Depression and listlessness can also aid in possession, as in cases of acedia.
The spell penetrate disguise will reveal the presence of a demon within a possessed creature.