They didn't understand germ theory at the time. They had no clue that boiling water would make it potable. If they had known that, so many lives would have be

They didn't understand germ theory at the time. They had no clue that boiling water would make it potable. If they had known that, so many lives would have been saved. What they did understand was, if you made beer the "wrong way", it would go off. Turns out the "right way" to make beer is to eliminate any competition for the brewers yeast.