We need some 'nastyness' from nature unless we want to become sick otherwise.. 'clean' isn't always a good thing unless we have open wounds, or the contaminant is particularly aggressive. Our whole body has evolved in raw nature, so we need a balance where we avoid the most dangerous, and allow everything else - to some extent.

"However, they cautioned that the results were based on a single trial day at each site and that some participants reported swimming elsewhere during the study period."

So.. bullocks with standard 'the guardian' fear-mongering, sensationalism, and lacking journalism ?