It will be very subtle at first; so subtle that I won't even notice it. I probably saw one word, in passing... in this case, probably the word "chills," which triggered my subconscious memory of a song that contained the word. I don't actually remember seeing the word, because that's part of the insidious nature of this particular beast. I won't know where it came from. It will, suddenly, just be.
It's not until the song has done irreparable damage to the inside of my cranium that I even become aware of its presence. By this time, of course, it's too late. The song has taken hold of my head. I am no longer in control of my thoughts; the song is now in control. All I can do is try to drown it out with something better, or stab myself in the ear with an icepick.
Since I rarely actually have an icepick on hand, however, this is usually when I tweet some lyrics and infect at least 10 of you, too. HA HA, suckers! Why should I suffer alone????