There is a school of thought that says that 1000+ years ago, clean water may have been hard to come by in some areas, and it was actually safer to drink beer than the local water.

It turns out, though, that much of the benefit comes from boiling the water as part of the beer making process - if those people had simply boiled their water before drinking it, it would have been just as effective.

The beer they drank back then was also much weaker than beer today. So nobody was getting drunk on it regularly, and it was not quite as dehydrating as modern alcoholic drinks.

The Royal Navy famously gave beer and rum as part of their rations while at sea, because the alcohol served to kill microbes. They could dilute it with water before handing out the rations for the day, and the alcohol would kill any microbes in the water, essentially helping them to stretch out the life og their water rations as well.