morning sickness vs sickness

morning sickness and sickness both are nouns.

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

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

morning sickness: nausea early in the day; a characteristic symptom in the early months of pregnancy
sickness: the state that precedes vomiting

Other hypernyms of morning sickness include nausea.

morning sickness (noun)	sickness (noun)
nausea early in the day; a characteristic symptom in the early months of pregnancy	impairment of normal physiological function affecting part or all of an organism
	the state that precedes vomiting
	defectiveness or unsoundness