It was a board-wide thing that happened in November playtime last year

It began at a Ball that was held at Hogwarts. Lots of people sick at the Hospital. I think it was a month or two before there were any cures because the sickness had been created by an ex death eater killing himself and getting back at the world kind of thing.
So it affected people's magic and so on. And anyone there could have potentially gotten it. There were quite a few deaths, but mostly people were just sick for a long time before getting better and the symptoms varied a little between patients.