My understanding is that disease is a form of negative karma. When a saint heals someone, what happens to that negative karma?
Karma is energy that a person has put into motion and is coming back to that original agent. It is we who think of it as positive or negative. In reality, it is merely a force that, by universal law, must complete itself.
Just as a strong person can carry some or all of a weaker person’s physical burden, often without apparent difficulty, so also a great saint can take on another person’s karmic burden. The karma hasn’t gone away; it is simply being drawn to the saint instead of back to the person who acted in the first place.
Sometimes saints endure great difficulty as a result of taking on many people’s karmic burdens, but they do not see it as suffering, because they are not identified with their bodies. They are happy to be able to help others in this selfless way, and to allow their bodies to be used by God as God wishes.