insanity vs mental disease

insanity and mental disease both are nouns.

Word	Noun	Adjective	Verb	Adverb
insanity	Yes	No	No	No
mental disease	Yes	No	No	No

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

insanity: relatively permanent disorder of the mind
mental disease: any disease of the mind; the psychological state of someone who has emotional or behavioral problems serious enough to require psychiatric intervention

Other hypernyms of insanity include mental illness, psychopathy.

insanity (noun)	mental disease (noun)
relatively permanent disorder of the mind	any disease of the mind; the psychological state of someone who has emotional or behavioral problems serious enough to require psychiatric intervention