I"m only a dabbler in historical knowledge, but in this case I think you may be looking for more of an anthropological or sociological answer. The thing is that even with modern germ theory a lot of this comes down to culture and in my limited experience is often cast through the lens of religion - Emile Durkheim immediately springs to my mind based upon your question with his focus on Taboo. Unfortunately, the only actual material I've seen which might qualify for /r/askhistorians for this type of question is of quite a dubious nature generally involving conjectures for why such-and-such group has whatever taboo which modern science tells us would have been a great way to avoid problems caused by issues not understood historically.

I might be misreading your question though as I'm really just focusing on the first half which is a wee bit broad and an actual Historian might be able to chime in with an answer to the second part for at least some parts of the world.