scarlet fever vs contagion

scarlet fever and contagion both are nouns.

Word	Noun	Adjective	Verb	Adverb
scarlet fever	Yes	No	No	No
contagion	Yes	No	No	No

As nouns, contagion is a hypernym of scarlet fever; that is, contagion is a word with a broader meaning than scarlet fever:

scarlet fever: an acute communicable disease (usually in children) characterized by fever and a red rash
contagion: any disease easily transmitted by contact

Other hypernyms of scarlet fever include contagious disease.

scarlet fever (noun)	contagion (noun)
an acute communicable disease (usually in children) characterized by fever and a red rash	the communication of an attitude or emotional state among a number of people
	an incident in which an infectious disease is transmitted
	any disease easily transmitted by contact