I'm building a world where a small subset of the population have the ability to induce rapid healing (potentially capable of sealing a large wound in seconds) by encouraging rapid cell growth. However, any fast healing has the potential of creating a long-term disease called "overgrowth", which is literally just cancer and has the same properties. The faster the healing, the greater the chances of overgrowth and the more severe the case will be.
There is no magic cure for overgrowth; further healing just makes it worse. It can be treated with focused death magic, but this has the same drawbacks as radiation therapy in our world (damaging to healthy cells, high potential for later recurrence).
What will be the effects of this form of rapid healing in medicine, warfare, and other aspects of society?
Other details about the setting:
This "magic" in this setting is relatively low-grade; it doesn't outright break the laws of physics and technically works by "communicating" with living cells and altering their behaviors and properties, sort of like genetic engineering on-the-fly. The energy/mass must come from somewhere, generally from food but many bio-mages can "cheat" by rapidly metabolizing nearby living organisms (like plants). Aside from the energy cost, there are no detrimental effects for the healer.
EDIT: To clarify further: Not all healing is dangerous; accelerating natural recovery in a clean hospital setting is virtually harmless. The faster the healing, the more severe the injury, or the more frequent the healing sessions, the more dangerous it becomes. Sealing an otherwise dangerous wound and allowing it to properly heal naturally is usually safe, but becomes more risky if it happens on a regular basis (sort of like x-rays). Completely healing a mortally wounded soldier in the course of combat so that they can continue fighting (as in, standard fantasy RPG healer practice) basically guarantees that they will die slowly and painfully over the next few years - but of course, an army that permits instant-healing soldiers is going to have a major advantage over non-healing ones. It's mainly intended to be a deconstruction of the "White Mage" style of combat support.