The mind becomes weak, they have a nervous breakdown by going mad and their beliefs get crazier.
People react to their suffering and call out for help and this is how they get possessed. An influx of demonic entities are attracted to suffering and by deceiving their host with projecting religious icons, through their Inner eye or giving them visions outside of themselves. (All Religions are Demonic)
People response to their suffering, by realising they put themselves there and they understand they need to get themselves out and then the help will follow.