| Musicosis is that particular phenomenon of obsession when a piece of music flips into your skull and takes root with the strength of an octopus on benzedrine. You'll spend the rest of the day humming the song, singing it, however badly; anything, just in an attempt to get it out of your system. You'll even try to hook yourself on new songs just to get rid of the current one
Sometimes this is brought on by hearing a fragment of the song. That fragment is a musicosis trigger.
I first heard the term musicosis used in an interview on the Westwood One radio network with David Lee Roth, late of Van Halen fame. He was describing how often this syndrome affects him and how much it drives him completely around the bend.
The only cure for musicosis that I've found (other than a Pan Galactic Gargle Blaster or equivalent) is to find a recording of the song in question, put on headphones, and listen to it like five or six times at high volume. The destructive interference of the song plus the imperfect copy in your head tend to cause just enough 'noise' that the song loses its phase lock with your mind and drops onto the floor. This can lead to frantic Napster, amazon.com, CDNow etc. searches looking for the song, as well as in extreme cases the feverish haunting of the wasteland of MP3 websites.
Danger, though; this type of therapy should be immediately discontinued once the cure is reached; otherwise, excessive use of therapy might lead to a stronger relapse based on a musicosis trigger in the therapeutic regimen.
Note: I just found the earworm node which describes the same phenomenon. My apologies; I hadn't seen that one. I'll leave mine up to denote there's more than one term for it... | 
