There are no such thing as stupid questions!
Mud fever occurs when bacteria make their way into small scratches and breaks in the skin around a horse's pastern or heel. The legs swell, get hot, and become very painful to the horse. My big gelding gets this once every couple of years. I try to nip it in the bud when I see the first signs but I usually end up having my vet out to give him antibiotics. This last year was horrible, as my barn burned, and I had to repasture quickly in the middle of winter. A wet year, and a new barn in a poorly drained field were a bad combination. Betadine scrubs can be helpful! Good luck! -M