Les vers dʹoreille: haunting music that runs through your head

Thanks to the Valais police forceʹs latest poster campaign "Libérez, dégivrez", you may have had the Snow Queen song stuck in your head all day long. If so, then youʹve been the victim of an earworm. But whatʹs an earworm, neurologically speaking? Are there more dangerous melodies? How to get rid of these musical parasites?