Is it possible for these old remedies to matter today? For an investigation into this and for a story worthy of HN to boot listen to / read the transcript of this RadioLab podcast.
That episode was pretty interesting, and it gives a whole new perspective on traditional medicine and science.
TLDR - a microbiologist and a historian decided to try a very old cure for a stye (staph infection in a pore of the eyelid). After brewing it and testing it several times it proved to be extremely effective against staph (including MRSA).
Why did the cure stop being used? Likely the same problems we're facing today - resistance. After nearly 1000 years of disuse, the cure has become effective again as staph has lost that specific resistance.
As an historian of early modern medicine I absolutely love the historical component of this story and I trust the microbiologist's expertise regarding the efficacy of the cure. But I'm extremely skeptical about the resistance claim. First off, what evidence do we have that this cure was ever widely used in Anglo-Saxon times? We'd require not just its presence in Bald's Leechbook but the 9th century equivalent of actual treatment records, or some kind of material/archeological evidence like remains of this specific cure in drug jars (which so far as I know don't exist). Second, Anglo-Saxon England was basically a backwater in terms of the total human population of the time and was not particularly well integrated into larger global networks. If the cure was present in a Tang Chinese text or even something from northern Italy or the Byzantine Empire, it would be more convincing that it would have had far-reaching epidemiological effects, but with the existing evidence I think it's a huge leap to assume that it actually was used to treat enough people successfully that it resulted in widespread resistance.
Anyway, I still really enjoyed the Radiolab piece about this (I love that they met at a Viking re-enactment!) but I think the implications of this have been really distorted in the popular press. We'd need to know an order of magnitude more about the global disease environment in Late Antiquity to actually make any guesses about the impact of this cure at the time it was set down.
http://www.radiolab.org/story/best-medicine/