Most commoners would see this as the skill that deals with the care and application of leeches, but through trial and error ("hey, he drank that and didn't die"), there's been a slow accumulation of medical practices by the scholars of Vellichor. One doesn't need to be a priest of the pantheon or a related scholar to be trained as a healer, though there's a common stereotype that the vast majority of healers are. Characters with this skill can improve the healing of other characters, and help them not die from infections with some plausible possibility of success.
The knowledge and science of diseases and treatment in Arx can be summed up by the basics of these real world theories: humoralism (black bile, yellow bile, phlegm, blood) and the Miasma Theory of Disease. Germ theory is not a thing.
0: Is it feed a fever and starve a cold, or starve a fever and feed a cold? I dunno try both.
1: Oh, it's actually humors. That's right. All the biles. ALL OF THEM.
2: And there's this plant that helps with that cough. Cough-plant. It's great.
3: Oh, cracked ribs. I can set those, and you won't die from internal bleeding. Hurray!
4: Desperate people travel from near and far to get your help.
5: People that really should be dead miraculously survive over and over again.
6: Resurrection hasn't been completely ruled out.