Give us this day our daily bread

Daily.

Like a daily vitamin.

Bread sits out to rise, collects all the bacteria and other nasties floating around in your enviro.

Gives them a little flour to eat, then cooks them all dead.

You eat the dead stuff in the bread and are inoculated, just like with a real vaccine.

Same kind of benefit as eating local honey, where the bees kill all the critters.

https://www.webmd.com/diet/sourdough-bread-good-for-you