illness vs sickness

illness and sickness both are nouns.

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

As nouns, illness and sickness are synonyms defined as:

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

Other synonyms of illness include malady, unwellness.

illness (noun)	sickness (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