Rudellian brain fever is a disease.
In 2373, Miles O'Brien considered the possibility that Keiko O'Brien was suffering from Rudellian brain fever from an outbreak on Bajor after claiming to have be controlled by an entity that O'Brien later learned was a Pah-wraith. (DS9 episode: "The Assignment")
Andrea M'Benga thought she might have contracted Rudellian brain fever after collapsing on Vulcan in 2374. Julian Bashir diagnosed M'Benga's collapse as being caused by severe exhaustion brought on by overwork, not Rudellian brain fever. (TOS novel: Dark Victory)