There is almost no possibility of cross contamination in the sense you think. What is scary though is that a lot of older cities and metropolitan areas are on what is called combination sewers. Basically, sewage and storm water runoff go into the same sewers and are dumped into waterways with little to no treatment. Back when the sewers were laid the population density was so sparse that it was no big deal to do so. Now the cities have grown up and with high rises and skyscraper office buildings stacking people on top of each other and now that it has become an issue there is no way to fix it without tearing buildings down. Now it sits there, a silent enemy.
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