ague vs illness

ague and illness both are nouns.

Word	Noun	Adjective	Verb	Adverb
ague	Yes	No	No	No
illness	Yes	No	No	No

As nouns, illness is a hypernym of ague; that is, illness is a word with a broader meaning than ague:

ague: a fit of shivering or shaking
illness: impairment of normal physiological function affecting part or all of an organism

Other hypernyms of ague include malady, sickness, unwellness.

ague (noun)	illness (noun)
a mark placed above a vowel to indicate pronunciation	impairment of normal physiological function affecting part or all of an organism
a fit of shivering or shaking
successive stages of chills and fever that is a symptom of malaria