Chronos:
The compound in willow bark is salicylic acid. Aspirin is acetylsalicylic acid, a closely-related compound derived from it. Both have similar benefits, but the side effects from salicylic acid are much worse, especially on the digestive system. And it’s not easy to process salicylic acid into acetylsalicylic acid.
Likewise, it’s easy to get bread mold, but that does you very little good. One particular strain of one particular species of bread mold produces a very safe and effective antibiotic. Other strains or species of mold produce other substances, some of them more or less similar to that antibiotic. Some of those substances aren’t safe to humans, some aren’t very effective versus bacteria, and some are neither. And I don’t think it’s even known how common the “good” strain is in the wild: It’s very common now due to us going to great lengths to cultivate it, because it’s so useful to us, but we don’t know exactly how lucky Fleming was to have that particular strain growing in his petri dish.
Don’t read too much into the fact that premodern doctors used moldy bread in poultices, by the way. They did, but then, they used just about everything imaginable in poultices, including various sorts of dung. It’s conceivable that some few patients in history might have gotten lucky and ended up with the good strain of penicillium in their moldy-bread poultice, but overall, people would have been much better off with poultices of just boiled cotton cloth with no mold (or honey or dung or whatever) at all.
When i wrote that, i was envisioning our Doctor member going back in time.