> You could boil plain water and be safe, but that wasn't understood until later Not sure that's true. We have evidence that ancient Egyptians, Greeks and Roma

> You could boil plain water and be safe, but that wasn't understood until later Not sure that's true. We have evidence that ancient Egyptians, Greeks and Romans (and I assume other societies) understood that boiling water made it safer to drink. Basic sand/gravel filters. Hypocrates for instance described a method of boiling water and then filtering it through a cloth to purify it and remove "bad smells". To be fair even if you have no clue about bacteria it's not a particularly complex concept to understand. If for instance pre modern people truly drank beer because it was safer surely at some point one of them would've asked the question why is that the case?