heh, nice question.
Many of us have spent time beside a pool. And you have probably wondered: what are the odds that no children (or adults, for that matter) have peed in the water? When pressed, we'd have to admit that the odds that the pool is pee-free are close to zero, but the lack of absolute certainty allows us to relax and swim anyway. We may comfort ourselves with some fuzzy thought about chlorine, or the immense volume of the pool relative to a few bladders, and our concerns slip away.
When things are very close to being certain, but we are still able to pretend otherwise, we are experts at using this window, small though it may be, and expanding it. For example, lots of people don't wash their hands after visiting the lavatory. We all know this, but we can happily imagine that everyone who cooks and serves in a restaurant we patronise does. At least until we see a server leave the stall, straighten their shirt in the mirror, and walk out without so much as a rinse. Dinner is served ruined! It's only when we face direct evidence that we can no longer put our heads in the sand.
Quite an interesting psychological insight into our behaviour, no? But then, the follow up question is, how many people will start washing their hands after seeing that behaviour? Take my own example, I have pee’ed into the pool when I was a child and teenager and then stopped. Washing hands after taking a pee? only idiots who didn't wash their penis’s and keep it clean and manage to get pee over their hands need to wash. Or so I thought.
Its only much later that I realise that the reason you wash your hands is not because of the bacteria on your wee or penis, but because more we wash, the more the overall toilets are cleaner. You have a bigger chance of catching an illness from the taps, the door handles, etc. etc. than from having a few spots of wee on your hands from your tinkle..