fanatic
1. [ noun ] a person motivated by irrational enthusiasm (as for a cause)
Synonyms: fiend
Examples:

"A fanatic is one who can't change his mind and won't change the subject"--Winston Churchill

Related terms: enthusiast
2. [ adjective ] marked by excessive enthusiasm for and intense devotion to a cause or idea
Synonyms: overzealous rabid fanatical
Examples:

"rabid isolationist"

Related terms: passionate
Similar spelling:   fanatical
  fanaticism
  fantastic