When you're an enthusiast, you're all jazzed up about a person or cause. A sports enthusiast is someone who is really passionate and excited about sports.
Enthusiast comes from the Greek enthousiastēs, meaning "person inspired by a god." While it doesn't retain those same religious connotations today, an enthusiast is someone who is inspired by a cause or person. A candidate running for office will have a group of enthusiasts working doggedly on his campaign. Often your mother is your biggest enthusiast or supporter. Non-profit organizations are always looking for enthusiasts who can make donations and become patrons of their organization.