Bieber Fever is a disease caught mostly by TWEENAGE girls, caused by overdosage of Justin Bieber music, merchandise and videos. You know you "have Bieber Fever" when you have more then one Bieber poster, you have at least one album, know the words to "Baby" off by heart or went to see his movie "Never Say Never" in the theater.
Girl 1: Oh my god, isn't Justin Bieber so hot?!?!
Girl 2: Oh my god, I know!! I have Bieber Fever!!