sickness vs illness

sickness and illness both are nouns.

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

As nouns, sickness and illness are synonyms defined as:

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

Other synonyms of sickness include malady, unwellness.

sickness (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
the state that precedes vomiting
defectiveness or unsoundness