Natural springs are rare, while streams and rivers are often muddy, seasonal and/or contaminated by domestic animals and wildlife. There's a reason wells were very common.
Well water is not safe either. It's exposed to the atmosphere, so to dust, birds (both s(h)itting and flying), and insects, etc.
Depending on the shape and structure of the well, rats can crawl inside, and lizards, and cockroaches and other insects can crawl in anyway, and poop there. and we all know about rats as a vector of many serious diseases, like the plague in said mediaeval times.
Depending on the shape and structure of the well, rats can crawl inside, and lizards, and cockroaches and other insects can crawl in anyway, and poop there. and we all know about rats as a vector of many serious diseases, like the plague in said mediaeval times.