Similarly, I grew up spending most of the year in California but summers in southern Mexico, and the first week or two of every summer was an acclimation period of intestinal distress. (This is despite never drinking unboiled tap water. There are just different microbes floating around in the environment.)

On the other hand, some of the pathogens you can get from raw milk (or insufficiently treated water) can be extremely unpleasant/dangerous and there’s no sure guarantee you’ll get used to them.