Based on a conversation Lindsay and I had one night after watching a show about Box Jellyfish on Animal Planet. According to the show (not fact checking it right now) Box Jellies cause more human fatalities than any other animal (aside from humans, I'm assuming).
I think the show scared me a bit, which is funny because I don't live in the Box Jelly's home waters of Australia. I'm never going to have to deal with these guys. It's not like they'll float out of the ocean on an air current and be waiting from me at my second story apartment... right?