In some cases, the intruding energy causing disorders and illnesses is not a simple entity looking like a bug
or a reptile, but a more complex spirit, such as a dead person's soul.
Sometimes these spirits just have nostalgy of the embodiment.
Indeed, although many neo-shamans regard spirits as a product of our own will and of an "impersonal energy of the Universe", actually spirits have personal goals and wishes, the most frequent of which is the wish for an embodied life.
That's why Power Animals want to be danced by shamans: in order to enter their body and possess it during the ecstatic dance. If their wishes are not met they will go away in search of a new human ally.
However some spirits aren't contented by a sole dance and want to live a true incarnation, hence they will seize possession of a body having a weak soul.
Some entities do have a sort of mind structure, but no full personality: they just contain behavior patterns or thought
patterns, in a word a partial personality.
I refer to such entities as demons.
They are guest spirits half-way between intrusion and possession.
Human consciousness is too complex to handle and a demon cannot control it completely or can just do it for a very short time.
I happened to cure a man who was gentle and helpful with his friends, but when someone did something wrong, even a little thing, he would change attitude abruptly: he would become quite a different, hard person breaking off relations with the "guilty" friend.
The breakup was as effective as to erase all previous friendly feelings: after the event he would no longer feel hate, just total indifference. Even if he later realized to have exceeded in his reactions, he was no longer able to resume relations.
He met his guest demon in a dream: it appeared like a stout man without eyes.
One of the missing eyes was replaced by a horrible metal plate inserted in the skull and provided with a telephoto lens.
The demon explained that he was his bodyguard. He added that by using the metal protesis he could not see anything, he only could take aim with uppermost precision.
As an istance, he pointed a woman coming down a stairway (the man had just argued with a woman friend). She was very far away and in motion, but the weird telephoto stirred, locked the target and fired, hitting the woman in the heart.
In this case the bodyguard demon had the task of protecting the host man's heart, which an old soul loss had made very fragile.
Demons are symbiotic beings, which are called on by our needs. Faithful servants hard to get rid of.
They will accomplish some simple tasks in standard situations. They may stay asleep for years until the standard situation occurs again and wakes them up.
Some demons may be helpful in some life stages, however we should break the alliance sooner or later. Increasing awareness requires us to get rid of all demons little by little, which is one of life's hardest tasks.
Other more harmful demons induce their hosts to repeat a certain wrong behavior or are cause of obsessive-compulsive disorders.
Demons may spread from person to person: you have an evidence of it when some compulsive or violent behaviors become contagious.
It's hard for a shaman to expel demons because they tend to return to the host who has been feeding them to date.