A musicosis trigger is a lyric or melody fragment (or even rhythm fragment) that serves to pound an entire song into your head in such a way that you can't get it out for at least twelve hours. Now you're walking around determinedly singing or humming this song, or maybe trying it in harmonics just for variety, and eliciting looks of pity and disdain from those surrounding you as you wander by with melodic muttering in your head-

Musicosis. Now you've got it. It's a condition that affects people with strong memories and associational tendencies. Now, unless you want to risk the cure, there's nothing to do but wait.

And hope you don't trip over another trigger.

Some lines I've known to function well as Musicosis Triggers

...there are millions more, but you may get the idea.