epilepsy vs brain disease

epilepsy and brain disease both are nouns.

Word	Noun	Adjective	Verb	Adverb
epilepsy	Yes	No	No	No
brain disease	Yes	No	No	No

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

epilepsy: a disorder of the central nervous system characterized by loss of consciousness and convulsions
brain disease: any disorder or disease of the brain

Other hypernyms of epilepsy include brain disorder, encephalopathy.

epilepsy (noun)	brain disease (noun)
a disorder of the central nervous system characterized by loss of consciousness and convulsions	any disorder or disease of the brain