One of my party’s characters recently rolled on the Corruption table (after character burning, during the campaign) and got the Diseased trait. There are several worse than that such as Maimed or Blind. Unfortunately, Burning Wheel doesn't seem to have much in the way of magical healing... Is there any mechanical way to either get rid of the trait or reduce the mechanical negatives from traits such as Diseased?
The standard way to get rid of a trait is to have it voted off by the group during a trait vote. For that to conceivably happen, the player has to work on getting rid of and acting against their trait over the course of many sessions. In principle, a Miracle courtesy of Faith could possibly also provide a drastic enough change to the character that voting off the trait makes sense.
What you've got in this case, though, is Corruption. This is what you get for calling on dark powers. It's not supposed to be a good time.
Some of the traits are not particularly nice. Some are not harmful, nor even useful. The character is stuck with them, though.
In all honesty, it's probably best for the player to embrace this change in their character and play it out to its conclusion.