I believe that the following scenario is comparatively rare in science fiction: humanity has developed viable interstellar travel and has discovered habitable planets but has not colonized any of them because of a static social structure. The social stasis of Jack Vance’s To Live Forever has been produced by a combination of artificial intelligence central planning and population control. A relatively small geographic region of a future Earth has sealed itself off from barbarous tribes and has separated its population into strict castes: brood, wedge, third (arrant), verge, and amaranth.
“Liane sped down a wide avenue lined with a few stunted old cypress trees, and he heard him close at his heels. He turned into an archway, pulled his bronze ring over his head, down to his feet. He stepped through, brought the ring up inside the darkness. Sanctuary. He was alone in a dark magic space, vanished from earthly gaze and knowledge. Brooding silence, dead space…
He felt a stir behind him, a breath of air.
At the end of The Eyes of the Overworld, Cugel finds that Iucounu has undergone some changes. A creature from Achernar, kin to his own daimon Firx, has infected him. Now Firx, residing in Cugel’s liver or thereabouts, is unable to control Cugel’s behavior except by the negative reinforcement of internal excruciations. Iucounu, however, has managed to have the creature take over his nervous system.
The parasite uses his host to communicate with Cugel in some odd ways.