So if the choice is between unsanitary water or sterile and containing chlorine, etc., which has the effect of strengthening the microbiota so that the resulting population increases perceived wellness in the host organism’s sociopolitical fitness, its simple enough, we all stop eating and drinking for ultimate safety and maximum paperclips.
You have a back door as well for the gut that tends to open at least once a day. We know fecal matter is in the air in most bathrooms. It wouldn't surprise me if you got an infusion of community fecal biome right into your anus every time you used a restroom that isn't your own.
Its not like there is positive air flow keeping any microbes in the air out and away. If something lands on there it can probably colonize then spread to the inside slipping through the sphincter.
