nettle rash vs roseola

nettle rash and roseola both are nouns.

Word	Noun	Adjective	Verb	Adverb
nettle rash	Yes	No	No	No
roseola	Yes	No	No	No

As nouns, roseola is a hypernym of nettle rash; that is, roseola is a word with a broader meaning than nettle rash:

nettle rash: an itchy skin eruption characterized by weals with pale interiors and well-defined red margins; usually the result of an allergic response to insect bites or food or drugs
roseola: any red eruption of the skin

Other hypernyms of nettle rash include efflorescence, rash, skin rash, hypersensitivity reaction.

nettle rash (noun)	roseola (noun)
an itchy skin eruption characterized by weals with pale interiors and well-defined red margins; usually the result of an allergic response to insect bites or food or drugs	any red eruption of the skin