tarotgal

Hmmm...maybe you could make it Cho Chang, if you want a Ravenclaw, though I know she doesn't show up all that much until book 4. She plays Quidditch, right? You could fall back on the 'practicing-in-the-rain' idea.

Another thought is maybe the whole thing could be started as a Weasley twins' prank. Patient zero could be the random student unfortunate enough to pick up the rigged sweet. Could also be a magical accident of some sorts. Not sure I like this idea as much as a natural illness though...just a thought.