At http://joannenova.com.au/2015/07/lost-knowledge-10th-century-cure-for-mr… … we learn that a medieval cure by a so called 'leech doctor' was found to treat superbugs that modern doctors struggle to cope with. A recipe for an 'eye salve' surprised researchers when it worked against the superbug MRSA which is apparently resistant to modern antibiotics. The recipe came from Bald's Leechbook and calls for garlic and onions, wine, and bile from a cow. The mix had to be brewed in brass and left for ten days.
Researchers at the University of Nottingham followed the recipe as closely as possible and then tried it out on some guinea pigs. Presumably in the ten days there was a certain amount of microbial growth as antibiotics were developed from fungus such as penicillin and cephalosporine which seem quite able to kill competitors and pests. The researchers themselves suggest the magic is in the slow chemical process.