fever vs hyperpyrexia

fever and hyperpyrexia both are nouns.

Word	Noun	Adjective	Verb	Adverb
fever	Yes	No	No	No
hyperpyrexia	Yes	No	No	No

As nouns, hyperpyrexia is a hyponym of fever; that is, hyperpyrexia is a word with a more specific, narrower meaning than fever:

fever: a rise in the temperature of the body; frequently a symptom of infection
hyperpyrexia: extremely high fever (especially in children)

fever (noun)	hyperpyrexia (noun)
a state of nervous excitement	extremely high fever (especially in children)
a rise in the temperature of the body; frequently a symptom of infection