the potential for sarcasm here is almost overpowering however note my restraint I would like this to be an example for other posters
a lesson in forum etiquette
No need to pat yourself on the back too much. I was merely biding my time until the person who made that statement to overextend their claim. I.E. YOU SPOILED MY FUN!!!
Even though it would have depended on whether one was able to access artesian wells or water table wells and to what extent run-off and (at the time) sewage seepage or agricultural seepage infected those supplies, it still would be laughable to suggest that's the only way people rehydrated themselves.
Even if you just accessed lake water or river water, it's likely they would have been able to discern which sources proved safer through natural filtration alone. On top of all that, the people of one geographic area get used to commonly occurring bacteria rather quickly. The problem arises in algal blooms and, as you mentioned or sudden or uncommonly large ecoli source contamination (something which can also occur during a fish kill due to an algal bloom or untimely turnover).
it is the fermentation process which kills off the nasties not the mixing
The mixing dilutes the contaminants hence there being "safe" levels of contamination accepted by most national health and safety organizations. The mixing, could also kill off contaminants if the right chemical reaction took place. That's why adding some chemicals to water disinfect it.