I guess how one defines dirty comes into play. For me dirty water is turbid water, cloudy, contains organic material it just looks not clear. It is the clear water that looks the same day after day that we assume is safe because we put faith in the supplier.
E.Coli 157, Giardia & Cryptosporidium are three strains of bacteria that quite happy live and multiply in "clean clear" water. You would have no reason to suspect you are ingesting them. There is no taste or odor. The only way to control their population is to add enough disinfectant to the water to kill them and to ensure that the distribution pipes are a clean and a sealed delivery system. The levels of residual disinfectant throughout the entire water system have to be checked and maintained several times a day.
Some strains of E.Coli such as 157, are quite capable of killing a person.
https://en.wikipedia.org/wiki/Walkerton_E._coli_outbreak