eruption vs exanthem

eruption and exanthem both are nouns.

Word	Noun	Adjective	Verb	Adverb
eruption	Yes	No	No	No
exanthem	Yes	No	No	No

As nouns, exanthem is a hyponym of eruption; that is, exanthem is a word with a more specific, narrower meaning than eruption:

eruption (noun)	exanthem (noun)
the sudden occurrence of a violent discharge of steam and volcanic material	eruption on the skin occurring as a symptom of a disease
a sudden very loud noise
the emergence of a tooth as it breaks through the gum
a sudden violent spontaneous occurrence (usually of some undesirable condition)
(of volcanoes) pouring out fumes or lava (or a deposit so formed)
symptom consisting of a breaking out and becoming visible