I recently adopted 2 very very sick bps with terrible scale rot. I mean, when I held the thing for the first time my gag reflex was kicking in strong.
The first thing I did was put them in tubs with paper towel as substrate, which I changed very regularly, and did not give them water enough to soak in. I used polysporin on them twice a day until their first shed, which was in about 2-3 weeks, then once a day until they shed again, which was very quickly after that, maybe 1-2 weeks. (I can't remember if both of them shed the second time....) I am guessing that not all cases are like this, but that is how it went for these two lucky serpents. After that, I kept an eye on them and they have not exhibited any signs of it returning.
If the substrate is too wet and it is not changed every month or so, it is possible that it is scale rot. It may also be possible that the belly is red because of a burn, or humidity that is too low (I think). Some bps have red bellies right before they shed too. You could post pictures of it so that people could give you a better idea.