social disease vs contagion

social disease and contagion both are nouns.

Word	Noun	Adjective	Verb	Adverb
social disease	Yes	No	No	No
contagion	Yes	No	No	No

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

social disease: a communicable infection transmitted by sexual intercourse or genital contact
contagion: any disease easily transmitted by contact

Other hypernyms of social disease include contagious disease.

social disease (noun)	contagion (noun)
a communicable infection transmitted by sexual intercourse or genital contact	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