I'm not sure, really. We can only look at the natural world around us to compare, and in nature, everything is built around energy transference. Literally everything is in the act of transferring energy from one point to another, from the universe itself, all the way down to the zero spactime nodes that make up quantum "froth."
Demons, I have always thought, operated on a similar principle of energy transfer at a level we can not really perceive, except perhaps through the use of synthetic or naturally-produced substances, such as DMT (the use of pharmakaia, or sorcery as a seer would). So someone who is spritually vulnerable due to choosing to harbor negativity chronically have something in the make up of their soul that demons find it easier to stick to and feed off of. Otherwise, why just seek our destruction? That's too simplistic an explanation. They're not just seeking our destruction, but must feed on the negativity as well.
So if we know demons are negative-oriented beings, their presence would radiate that negativity to others, and those with a brain apparatus primed for it would "tune-in" to this demon's energy and reflect the nature of it without realizing what was happening, all the while the demon is riding them like a parasite. We know that demons are can physically manifest--the pigs drowning themselves is evidence of that. But why the pigs? Why not other animals? Why not other beings? Simply because, I believe, that the demons couldn't just float about--they had to be in something or they'd need to go right back to where they came from.
Which means that in today's world, there's an all you can eat buffet of negativity that demons are undoubtedly having a party with. Revelations 9:2 indicates that Satan releases all his buddies for a time, which will likely be a time of horror unlike anything we've seen so far, because at that point, the fallen angels are free to run around the earth again. We also know that our awareness increases at the end as the Holy Spirit is poured out on the world full-force.
This is what seems to line up; however, my heart is a lying POS because I'm human, so I still seek to corroborate everything and as I go along my path of learning, it is likely that my opinion will change to reflect the in-dwelling of God.