enthusiast, n. [http://oed.com/view/Entry/62880]
3.a. One who is full of "enthusiasm" (see enthusiasm n. 3) for a cause or principle, or who enters with enthusiasm into a pursuit.
enthusiasm, n. [http://oed.com/view/Entry/62879]
3.a. Rapturous intensity of feeling in favour of a person, principle, cause, etc.; passionate eagerness in any pursuit, proceeding from an intense conviction of the worthiness of the object.