Note: this is not a question about history, but about human digestive system over time :)
I know a lot of colleagues who traveled for business trips to India. All of them caught terrible diarrhea there, in spite of keeping all the rules related to use of reboiled or canned water only, eating only peeled fruits, not eating the street food etc.
Did they have to keep the same safety dietary rules as the contemporary Westerners all the time? (I cannot see this as technically possible...)
Were our intestinal flora and fauna different one hundred years ago?
Was the hygiene level and quality of water much lower in Europe then so coming to India was not such a biological shock? (Well, I would believe this in the 1800s but not in the 1900s...)
Or did just some of them die and the rest got accustomed?
I am aware that there is an initial shock for your body when travelling to a very different area generally. However, the Indians (and other Asians) can easily drink the tap water in most of the EU countries and eat the street food, which is not valid vice-versa. So there must be some difference.