@cryum
By receiving the same dose in time lapses. something like drinking alcohol, you(your body) eventually gain some resistance to alcohol. Of course that applies to medicine and sickess too.
@Reimans that's only "normal" for poison. The average medieval person does not consume enough medicine in their entire life to develop a resistance. Who does that?
Of course they don’t know anything about viruses or their existence. They weren’t discovered until the late 18th century. You were pretty much given the best skill a guy could ask for in a world like that.