Ancient wisdom often sounds like common sense now that it is commomly taught. What is some ancient wisdom that we no longer teach because it was wrong?
I don't know how common it is but my grandmas always said that you shouldn't eat pork after being released from the hospital or while sick.
Then I finally remembered to ask a doctor about it and he said there's no such thing.
I understand that the notion behind it is to eat easy to digest foods, instead of red meat, in order not to burden your body trying to metabolise them while it is also fighting a disease.
I can sort of get behind this, but I also say you still need your protein. Over here a chicken stew is quite commonly given to sick people, maybe for this reasoning.
We don't digest cellulose, our intestines and appendix are not long enough. Compare our body plan to a horse or rat (giant appendix) or cow or sheep (multiple stomachs, filled with bacteria that break down cellulose). Cellulose goes right through. But that's the fiber that is always so highly praised and not actually a bad thing.
Could have been because of the prevalence of trichinosis in pork until relatively recently (i.e. in your grandmother's lifetime). If your immune system is in good shape, it's not terribly risky, but if you just came out of the hospital you probably aren't in peak form. So avoid pork for a while until your body can properly fight off parasites again.
Proper food safety and livestock handling has dramatically reduced the risk of parasite infections from pork in most of the developed world.