The irony that rotting plants would keep one healthy.
al·co·hol (lk-h?l, -hl)
n.
1. A colorless volatile flammable liquid, C2H5OH, synthesized or obtained by fermentation of sugars and starches and widely used, either pure or denatured, as a solvent and in drugs, cleaning solutions, explosives, and intoxicating beverages. Also called ethanol, ethyl alcohol, grain alcohol.
It kills bacterias, you see. THAT's why alcoholic beverages were popular back in the days.
Also, the 'rotting plants' aka yeast, is a very rich source of vitamin B and glutamic acid. Very healthy indeed.
Yeah, didn't really think about the fact that most of the water probably isn't going to be very clean...

Well, people used to take water from its very source, and not simply from the river. Spring water is clean, river water can be very poisonous. Otherwise, you can use the old philter method by using stones, sand and clay to rid the water from visible dirt, and cook it afterwards. I doubt people were only able to cook water since 200 years ago ... They probably mixed it with the food too, so that would get disinfected at the same time. Soup being a prime example of such dishes.
About milk: milk is very nutritious, also for bacteria. But these bacteria break it down to acid, which gives milk the sour taste when it 'spoils'. Which means a sour environment, which equals a very unsuitable environment for bacteria. In other words: milk will almost always be pretty safe to drink, as the bacteria themselves change it into an environment they cannot live in. It'll only taste bad.