You would be surprised how much weight you can lose from just an infection, especially if it's persistent.
Heck with an infection, try food poisoning. I had it once from a bad plate of fettucini alfredo and lost 16 pounds in a little over a day. Sure, I put a bit of it back on as soon as I could keep water down (was within an hour or two of going to get an IV for rehydration) but that was the quickest weight-loss method I've ever been through.