unwellness vs illness

unwellness and illness both are nouns.

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

As nouns, unwellness and illness are synonyms defined as:

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

Other synonyms of unwellness include malady, sickness.

unwellness (noun)	illness (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