disease vs sickness

disease and sickness both are nouns.

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

As nouns, sickness is a hypernym of disease; that is, sickness is a word with a broader meaning than disease:

disease: an impairment of health or a condition of abnormal functioning
sickness: impairment of normal physiological function affecting part or all of an organism

Other hypernyms of disease include illness, malady, unwellness.

disease (noun)	sickness (noun)
an impairment of health or a condition of abnormal functioning	impairment of normal physiological function affecting part or all of an organism
	the state that precedes vomiting
	defectiveness or unsoundness