hyperpyrexia vs fever

hyperpyrexia and fever both are nouns.

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

As nouns, fever is a hypernym of hyperpyrexia; that is, fever is a word with a broader meaning than hyperpyrexia:

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

Other hypernyms of hyperpyrexia include hyperthermia, hyperthermy, febricity, febrility, feverishness, pyrexia.

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