This bugged me for a long time, but overhearing this song while sick pushed me over the brink. What the hell is with Peggy Lee's "Fever"? There is nothing sexy or romantic about being sick/coughing constantly/having a running nose/feeling to weak to stand up/not having an appetite despite feeling hungry/headaches etc. It's a horrible metaphor. Also, who the hell thinks transmitting your disease to your SO is cool? (Kurt Cobain at least intended the I wish I could eat your cancer when you turn black line to be Sarcasm Mode...)
I think that here, fever = being HOT for someone else.
Okay, but that's still a pretty bad metaphor if you overthink it. Side-effects and all that.
Unless the writer of the song really doesn't know the end of Romeo and Juliet, I think that the side-effects of love are an evident sub-text in the song.
Fever is commonly associated with making the sufferer hot and sweaty, as is sexual arousal. There's also the term 'love sick' which says that attraction is a bit like an illness one comes down with.