Sorry, someone mentioned bidet and I thought of the classic bidet separate from the toilet.It sounds as if you may have missed the fact that this discussion has been about a "Japanese toilet seat", (almost a bidet, but for a somewhat different purpose) and, as for .....
.... Mr Wikipedia says ....
In view of my comments above, I don't think it's a matter of "knowing this" but, as I've said, I do think the 'risk' is not far from zero. The probability of backflow into the supply network occurring is very low, as is the probability of any contamination getting into the water at the bidet/seat, and also as is the probability of the individual concerned having pathogenic organisms to excrete. Multiply those three very small probabilities together and you get an extremely small overall 'risk '![]()
Of course, that Japanese seat which would get in direct contact with human waste is a potential hazard.

