We put a few spells for treating poison and disease on the Lay Healer's Blood Mastery base list in the current draft of RMU Spell Law. Conceptually, that would be about purifying the blood etc. It was odd that they have a whole base list for the niche function of replacing limbs but nothing for one of the most important functions of a healer in a medieval society.
As written in RMSS though, a lay healer would need to use medical skills and perhaps herbs to accomplish those things. They don't even have a general purpose diagnosis spell to identify the problem (we're adding that, too, similar to the Healer version).