I caught mononucleosis, though the reason why they wanted me to stay in hospital for a longer time was that I also got myocarditis, which means my heart could've failed big time if I did anything too crazy. My heart's getting better now though, and I'm probably getting out of the hospital soon, too. (right now I'm just on a one day vacation)
Thanks everyone! I've got out of the hospital now and I've got a couple weeks of vacation, so everything should be just fine. It was only the first few days that were more critical, but my heart's recovering now.