Fantasy setting does not equal "nothing matters, so anything goes". IMO, fantasy settings work best when the fantastic stuff is a specific exception, and the rest of the world operates more or less like ours. That grounding helps the fantastic "pop", so that the world is comprehensible.
Mummy rot as a magical disease works fine as a specific exception, IMO.
It's not anything goes. It's a recognition of the fact that the fantasy world does not have to conform to our world realism.
Magical herbs, if you don't want them in your fantasy world, ok. But to say they are not there because they're not realistic is preposterous.
Frankly if the world doesn't provide SOME evidence of it itself being fantastic (rather than just that guy there can pop a magic missile), that becomes boring for me. I mean, I can play a historical GURPS setting if I want my non fantasy fix.