Currently without human hosts. The ancient Egyptian hieroglyph system and the Gnostic Gospels are examples of "dead" schemes which lay dormant for millennia in hidden or untranslatable texts, waiting to re-activate themselves by infecting modern archeologists. Some obsolete memes never become entirely dormant, such as Phlogiston theory, which simply mutated from a "belief" into a "quaint historical footnote."
A meme that is currently inactive. Dormant memes can be inside human brains, inside computers or in the environment. A dormant meme that has infected its host is known as a latent meme. Synonym: inactive meme. Wikipedia has more details.