There is a vibrant field of medicine looking at "old wives tales" and "shaman cures" and whatnot to (a) work out if they do actually do anything and (b) how that happens.
I seem to recall an effort in India to write down all the tribal herbal knowledge in the country for this aim.
Some are a bit daft, though....
How about the old wives tale of having sex with a virgin to cure AIDS? :-)
It takes a lot of work to separate the wheat from the chaff