eli5: When people used wells for drinking water, how did they not get sick? Was there some type of filter or was the water just naturally clean?
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Other answers here are somewhat incomplete.
Old hand-dug wells did make people sick sometimes.
Drilled wells like you would find since the late 1800s generally did not because they are far deeper and the water takes years longer to filter down to that level.
It also matters what you are putting the well into.
A shallow well into the gravel of a spring is far safer than into gravel only holding water because the flow of the spring hold much of the potential contamination back and flushes it away if it does get in.
Even a drilled well is more susceptible to contamination if the rock being drilled is heavily fractured and the covering soil drains quickly.
Flood silt with a lot of organics mixed in like you find in many riverside cities will allow some potential contamination to thrive a lot more than rock, sand or gravel.
You can make deep or shallow wells that are safe, but doing so with limited choices of where to dig and no way of testing the water is a gamble at best.
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