brainworm

noun brainworms · brain worm · brain-worm

1. A neurotropic nematode parasite (Parelaphostrongylus tenuis). Only rarely do we find a brainworm-infected moose that has larvae in its feces.
2. Any parasitic, worm-like species that inhabits the brain of another organism, typically altering its behaviour or giving it special abilities. She caught my expression, and added quickly, “Oh, but not bad brain worms. Good brain worms. They know most of the languages that are spoken across the worlds, and pretty soon you will too.”
3. figuratively, informal A song or melody that keeps playing inside of one's mind. Which points out the difference between this song and other brainworms.

   Similar: earworm

4. derogatory, figuratively, slang, sometimes A persistent delusion or obsession; a deeply-ingrained or unquestioned idea. Of all the toxic brainworms Ronald Reagan passed down to his acolytes and imitators, almost none has proven more ineradicable and ruinous than his pathologically dissociative behavior when confronted with any information that threatened to impede his world-view.