fanatic
n. countablen. someone who has extremely strong beliefs or interests that other people might find too much. You use this for people who are obsessed with a hobby or a religion.
n. a person motivated by excessive enthusiasm and often uncritical devotion, particularly regarding religious or political matters. In modern usage, it frequently describes someone with an intense interest in a specific hobby or pursuit.
My brother is a real fitness fanatic who works out every day.
The movie star was followed everywhere by a group of fanatics who had waited hours just to see him.
First attested in 1525. Learned borrowing from Latin fānāticus (“of a temple, divinely inspired, frenzied”), from fānum (“temple”). Influenced by French fanatique.
Often used with a modifying noun to specify the area of interest, such as 'sports fanatic' or 'health fanatic'.