illness vs unwellness

illness and unwellness both are nouns.

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

As nouns, illness and unwellness are synonyms defined as:

illness and unwellness: impairment of normal physiological function affecting part or all of an organism

Other synonyms of illness include malady, sickness.

illness (noun)	unwellness (noun)
impairment of normal physiological function affecting part or all of an organism	impairment of normal physiological function affecting part or all of an organism