roseola vs nettle rash

roseola and nettle rash both are nouns.

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

As nouns, nettle rash is a hyponym of roseola; that is, nettle rash is a word with a more specific, narrower meaning than roseola:

roseola: any red eruption of the skin
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

Other hyponyms of roseola include heat rash, miliaria, prickly heat, hives, urticaria, urtication.

roseola (noun)	nettle rash (noun)
any red eruption of the skin	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