But not the same yeast everywhere. Plus bacteria are also a factor, and also vary place to place. Some may cause worse allergies than others, depending on the person.

As far as I know, French bakers didn't bring the yeast (or starter) responsible for sourdough. They just brought french bread. The sourdough part happened naturally, at least initially. Now it's actively encouraged.

But even with the right starter, the further east you go, the harder good sourdough is to make.