Demon

»But why are the demons involved in this?« she inquired. »Demons are never major characters. They care nothing about human events, unless they wish to interfere with them.«
»Will you make a princessly oath of secrecy?« the Professor asked. »The question has an answer, but it is not given for mere humans to know.«

Piers Anthony, Demons don't dream

Computers these days have daemons, as did Socrates, and surely these are related to demons. I have a hunch, in fact, that they are etymologically no different from demons, and perhaps there are no computer viruses, no hackers, no crackers, but only demons who have wheedled their ways into the favor of programmers and have been set free in the cyberworld to work their mischief. (I blame them, too, for my errors of typing.)

demons swarmed around her head like a cloud of angry gnats.

Demons walk among us, Part II. Don't forget parking valets, music critics,
exotic dancers, car salesmen, HR managers, circus clowns, and anyone who is insane enough to actually want to be president of the United States.

Clinically speaking, demons/daemons/devils are personifications of personal weaknesses/fear. To the perciever/believer they are quite real. Generally the demon is a symbol/metaphor for a personification of fear, they are made up entirely of fear. Their characteristic as demons points to a refusal to take responsibility for the demon, i.e. the devil made me do it.