The Fever Lyrics
H-E
F
E-V
E
R
Yeah, she's running on empty
She can't sing, she just screams and shouts
Yeah, she's running on empty
To live her life without you
She'd be the one to - she'd be the one to
Try and find what I'm after
Yeah, she can't sing, she just screams and shouts
But it's not easy
Because without him
My heart is empty
But it's not easy
Because without him
My heart is empty
Yeah, she's running on empty
She an't sing, she just screams and shouts
Yeah, she's running on empty
I don't know anything about you'
But she can't live - she can't live -
She can't live without you
Yeah, she can't sing, she just screams and shouts
H-E
F
E-V
E
R
Yeah, she can't sing, she just screams and shouts
(A joins in on third time)

Well I've never heard this from anyone else, but I know most of the songs on Pawn Shoppe Heart are bitching about SOMEONE, and to me this is just Karen O all over. Evidence! :
-The song is called "the fever." The debut album of the Yeah Yeah Yeah's (Karen's band) is called "fever to tell."
-"She Don't know what she's talking about." Kind of harks back to the title link again - what exactly is she telling us? I'm not a YYY fan, so it kinda resonates ;)
-"Yeah she's running on empty." I think this relates to her name, Karen O - that big old empty zero like O on the end.
-"She can't sing, she just screams and shouts." a fair accusation in the eyes of anyone who has heard karen's 'unique' vocal stylings ;)

Thats an interesting interpretation. ^_^
But I always thought this song was about some kind of love/hate relationship... i.e. "you piss me off like nothing else but dont ever leave me", sort of. And its a typical, common problem in relationships, and in this song its being referred to as "The Fever".