Cathy’s bought Zoo Puzzle for the PS2, where basically you have to match up animals into lines of three or more to make them disappear. Think Tetris with fauna and you’ll be there. It's also blummin' addictive.
Playing it in company inevitably results in people trying to help by telling you which animals you can line up next. Hence: "You’ve got hippos!" which sounds like a particularly awkward kind of ailment.
"Sorry, I can’t come in to work today. I’ve got a bad case of the hippos."