brainworm

A neurotropic nematode parasite (Parelaphostrongylus tenuis).

Only rarely do we find a brainworm-infected moose that has larvae in its feces.

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.”

"Yes. And I'm going to get back to her. Nothing's going to stand in the way of that. Not two thousand years...not some overambitious species that think they're gods...not monsters or brain-worms or secret assassins."

A song or melody that keeps playing inside of one's mind.

Which points out the difference between this song and other brainworms.

Although brainworms are no doubt ancient, the term has come into common use only in the last few decades, probably because suddenly music is ubiquitous, […]

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.

Venezuela's market-based economy looks nothing like the centrally-planned Maoist regime that Sean Hannity's brainworms tell him it is.