STD vs contagion

STD and contagion both are nouns.

Word	Noun	Adjective	Verb	Adverb
STD	Yes	No	No	No
contagion	Yes	No	No	No

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

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

Other hypernyms of STD include contagious disease.

STD (noun)	contagion (noun)
a doctor's degree in theology in the pontifical university system of the Catholic church	the communication of an attitude or emotional state among a number of people
a communicable infection transmitted by sexual intercourse or genital contact	an incident in which an infectious disease is transmitted
	any disease easily transmitted by contact