Science doesn’t believe in demons, but if you mean occult/4D science then it’s like this:

Our astral and etheric bodies are meant to allow just our own spirit to interface with our physical body. But that only happens if the body is strong and awake, if the etheric body is full of life, and the astral body is attuned only to spirit/divinity. I’m using old mysticism terms here because it’s a simple model that works well enough.

As long as these are all strong and you are fully here and present, then no simple demon or alien can possess you.

If the physical body is weak, then spirit has a harder time getting through to it. That can come from being on drugs, alcohol, sick, injured, or being tired or starving.

If the etheric body is weak, then likewise. That can come from being drained by energy vampires (by the demon itself or someone who is an energy vampire). It can also come from not breathing enough, from a weak body that fails to generate enough metabolic energy and therefore lifeforce energy, or not getting enough sleep.

If the astral body is weak or rather corrupted, then same. This comes from having a victim mindset, having no willpower, or the opposite of having dark and evil thoughts and feelings. Whether you resonate with predator mode or prey mode, either way you resonate with the demonic realm and less with spirit. That weakens the connection to spirit.

And if the spirit isn’t fully there in the body, same. This happens from being mindless, spacing out, mentally tired, or fragmented from having gone through trauma. An ungrounded, spacey person lacking self-awareness has an absent spirit.

So demons look for weaknesses to be able to get a foothold in a person.

Simple everyday example: in a moment of mindlessness a demon (or an alien) can exert a telepathic push to, say, make you forget to take your keys. So you get locked out of your home or car and have to pay a locksmith to get you back in. That’s the lowest level example.

The next step up would be to inject negative energy into you. You suddenly feel cranky and angry. This only happens if your astral body has enough latent anger and crankiness in it to be stimulated and activated. So already there must be some corruption in the astral body, which is normal for humans. So you get angry and say mean things to someone, and damage the relationship. Demon is happy it caused that.

Next step up would be the demon messing with your thoughts, emotions, and also sleep and dreams, and feeding on your energy. As a result you get nightmares, loss of sleep, and risk depression and irritability. Your physical body then gets weakened, etheric body too, astral body more corrupted as you shift into darker states of mind, and spirit therefore loses its grip.

Next step up is where the demon gains enough of a foothold into your etheric body that it can start messing with your brain directly. That means injecting visuals and voices and physical sensations like electric shocks and pokes and squeezes, which some victims may misinterpret as TI targeting by directed energy weapons.

So then you’d start getting schizophrenia symptoms. Or maybe it hovers about you as a dark shape offering to help you if you let it in. Or poses as a female ghost calling for your help, luring you into the attic where it has set up its nest. If you say yes, it then has permission (as given by angelic entities because you chose it) to jack even deeper into your being. So the voices increase and you lose your sense of independent thoughts and feelings.

Next step up is where it has almost 90% control of a person. The person’s consciousness feels far away, like a passive observer, while strong impulses and automatic thoughts/words/actions come out of them. The demon bas displaces the resident spirit and taken the wheel, so to speak. This only happens after a lengthy period of stalking and weakening by the demon of the intended target, and a bonding with the demon at the etheric and astral levels.

Full possession is when the person isn’t even conscious anymore, or has been kicked out of the body. Then the demon has full control and is looking through the eyes, speaking through the mouth, and living through the physical body of the person. Because demons are anti-life and have a toxic etheric energy field that kills plants and animals and humans, the host body will get sicker and sicker and die…unless it gets replenished with etheric energy from other living humans.

So this person is now a walking energy vampire. And presumably, if they reach elite status (like celebrities and politicians and secret society types) they may engage in ritual sacrifice, drinking blood of babies, harvesting their loosh energy through torture and so on, in order to maintain their youthfulness while having a highly toxic high level demon operating in their bodies. The adrenochrome lore is just the physical part. There’s a lot being harvested at a subtle energy level to feed the demon or reptilian.

Aliens in an out-of-body state can possess humans. Or just telepathically remotely influence them. Deceased humans, or their astral shells, can possess us too as hitchhikers. Usually there has to be an astral weakness or resonance that binds them with you, like if they were drug addicts were craved it even after death and you’re a living human who has a strong craving for it. Dr. William Baldwin’s books, and Dr. Shakuntala Modi’s books, are the go-to for that subject of entity attachments.