Gut flora: Partners share bacterial strains—and disease risks. Household members share gut bacteria; sweeteners negatively affect blood sugar https://www.ad-hoc-news.de/wissenschaft/darmflora-partner-teilen-bakterienstaemme-auch-krankheitsrisiken/69700866 The most likely mechanism is that we ingest a small amount of other people's feces https://www.apotheken-umschau.de/mein-koerper/immunsystem/mikrobiom-wie-das-zusammenleben-die-bakterien-beeinflusst-1544973.html