fanatic

nouns

a person motivated by irrational enthusiasm (as for a cause)
"A fanatic is one who can't change his mind and won't change the subject"--Winston Churchill
fanatic, fiend
is derived from
fanatical

adjectives

marked by excessive enthusiasm for and intense devotion to a cause or idea
"rabid isolationist"
fanatic, fanatical, overzealous, rabid
is derived from
fanatism