@OsoJr I mean, I'm both surprised and not surprised by the regression of medical knowledge.
On one hand, yes, no one knew about her powers, and the knowledge already existed. It'd only be natural that something that would save lives gets passed down.
On the other hand, all serious or fatal diseases (and perhaps even minor diseases as well) had practically been wiped out from the would for four entire centuries. Multiple generations of people. The people who originally lived through times where diseases were serious threats that needed much effort to treat and precautions had to be actively taken to avoid diseases all probably died within the first century. The generations who came after only had secondhand knowledge about the importance of disease prevention and treatment, and even those who entered the field of medicine wouldn't have firsthand experience on treating and researching said diseases because they had been wiped out by the saint. That in and of itself would decrease the value of the profession, and less people would be willing to go into such a field for a career as a result.
The longer people go without having to deal with a threat, the less vigilant they will be, especially if their grandparents, parents, friends, children, AND grandchildren almost never had to deal with the seriousness of the issue firsthand. As that much time passes, the importance and value placed on such knowledge by the people will naturally dwindle. In an age where knowledge was transcribed in parchment and books, the original documents would have deteriorated, and the transcribing of said knowledge that people have placed less importance on over several generations of records may become sloppier bit by bit, until it is completely lost.
