Generic ancient Sith war virus made up solely for narrative purposes that neither the Light nor Dark Sides on their own can completely heal or destroy, think of it as a way for whatever ancient Sith Lord who invented it to hurt both the Jedi and his rivals.  He hasn't cured it, he simply uses the Dark Side to attack and reduce its concentration in his daughter’s body before it can begin to do truly grievous harm to her. It’s not a cure, though, it’s a temporary solution and if he died his daughter would likely follow him within a week without treatment.

Lorn is slowly coming to the realization that it will need both sides of the Force to eradicate completely, and that he will need another Jedi Healer to assist him because even though he is really much more grey than dark, he can’t hold both sides at the same time.
It is one of the multiple reasons why he does not kill the Jedi he defeats or otherwise drag them back to Dagobah (sans Hrassk, because after their fight he wanted to become a Bogan) for interrogation/turning, because he does not want to alienate the Order too much. I'm working on updating my profile at the moment so you can see his other reasons for tending towards mercy rather than bloodshed.