Download syndrome is where you download things that you really don't want or need. You just love the thrill of downloading things, so you download stuff you couldn't give a shit about.
A: "Yo man, did you get that new fall out boy album"
B: "Yeah. Last night, I had a serious case of download syndrome. I picked up that album, and the new Britney Spears album. It's Britney Bitch!