'Demonic Possession' is a western construct. Immature and a naive approach to a very sophisticated method of personal/ group takeover. Taking over a body crosses genders, geographies and even species. Here in Bharat, possession is an art. A skill. A Siddhi. Who or what is possessed, when, how, for how long, for what purpose and the karmic consequence, is all carefully thought out.
The phases of the moon are an important ingredient in the possession mix. Certain nights are conducive for parakāya praveśanam. Siddhas and enlightened beings can enter and leave any body, living or dead. They can do it without even the awareness of the being. A certain siddha who is revered for his work in literature, poetry, medicine and inner technology once entered the body of a dead cowherd called Moolan. He noticed the cows of Moolan crying because the man they loved was dead. Taking pity on the poor voiceless beings, he entered the dead body, spoke softly and kindly to the overjoyed cows & took them home safely to their shed. Of course, the enlightened are plagued with the same issues that hound the ignorant. When he decided to leave Moolan's body, he realised that his body had been burned during his absence. So here he was, stuck with a new body.
It is not just with instances such as these. It is well known to many that Shankaracharya entered the body of a terminally sick king to explore the world of the Grihasta. As he was a bhramachari, he did not want to use his body to delve into the life of a married man. Deeply entrenched in a debate, Shankara wanted to get his facts straight, so shed his body to take on the body of a king.
In TamilNadu, this skill was studied to great extent and there were many people skilled in this art, a couple of centuries back. The art was called 'கூடு விட்டுக் கூடு பாய்வது', or the art of moving from one 'cage' to another. Kings were often taught this art as part of the art of war, to escape from enemies during perilous times.
Other beings try to enter the human body too. Vetalas are powerful beings who love to enter and animate the bodies of the dead. They do not displace any spirits and only use bodies at crematoriums, where the spirit has already left. Like the Vetalas, Aghoris are adept in using the bodies in the smashan to house powerful spirits to enable them to do their work. There is not much that many spirits can do without a body unless they do the necessary sadhana
People who have died violent deaths or have killed themselves can sometimes enter bodies of vulnerable people. Mentally sick individuals, women during their periods or just after childbirth, people with degenerative diseases that have left the body with very low energy and sometimes small children are most susceptible.