STD vs contagious disease

STD and contagious disease both are nouns.

Word	Noun	Adjective	Verb	Adverb
STD	Yes	No	No	No
contagious disease	Yes	No	No	No

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

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

Other hypernyms of STD include contagion.

STD (noun)	contagious disease (noun)
a doctor's degree in theology in the pontifical university system of the Catholic church	any disease easily transmitted by contact
a communicable infection transmitted by sexual intercourse or genital contact