sickness vs unwellness

sickness and unwellness both are nouns.

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

As nouns, sickness and unwellness are synonyms defined as:

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

Other synonyms of sickness include illness, malady.

sickness (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
the state that precedes vomiting
defectiveness or unsoundness