Thrax is a cytomegalovirus that has become lethal (because cytomegaloviruses are DNA viruses, and they exceptionally attack the brain) (Thrax wanted to steal Frank's DNA genome); Thrax is a human immunodeficiency virus (because Frank touched a chimpanzee before he caught Thrax). There was, for example, a section of Sword of the Lictor in which Severian is descending the mountains north of Thrax and describes a cliff face that cuts through strata of various eras and in particular describes some kind of gigantic geometric pattern that to me feels as though I am supposed to be able to figure out what he is talking about ... Including fleeing from Thrax and meeting the Autarch?

Understanding the Context

Those events can't have both versions happening at the same time while still being self-consistent. Or is Severian mistaken in believing this when he writes it in Citadel, before learning how stuff works in Urth? A changeling's strength comes from their ability to morph into stronger things. If a changeling doesn't morph, then it is limited to the physical constraints of the body that it is in.

Key Insights

In the fight between Sisko and Thrax, the changeling maintained the Thrax form until it escapes. I can add, as evidence, that the Federation and Klingons employed a blood screening as the sole method of ...