> People understand just fine what "illness" is. Every culture has a long tradition of medical practice since long before anyone knew what a germ was, and people in many civilizations (this isn't some eurocentric western-world thing) have known for hundreds or thousands of years to keep feces away from their food and water.
The germ theory is relatively recent and was not readily accepted in Europe. And for that matter Europe was very late to adopt a theory of keeping faeces away from their food and water.
Traditional Brahmanism in India is extremely fastidious. Sadly by connecting the practices to caste they did not become widespread among others.
Your sense of obviousness is inconsistent with the investigations of history or sociology.
Furthermore trying to tie everything to caste related issues will cause more harm than good, as it is popular and easy to blame a simple demon the truth is often more nuanced.
How true that is, and I agonized over how to word the end of my comment to avoid overstating/ misstating / offending / hitting peoples' hot buttons (pro or con). However I think there's an essential truth somewhere in there, and I chose the word "fastidious" because it is more clear than incorrect (and emotionally laden) words like "better" / "cleaner" etc. In fact "fastidious" can even have some slightly negative implications.
But to be realistic: my Brahmin grandparents and great grandparents (born in 1800s!) & family all lived well into their 90s as long as they survived childhood. My anglo grandparents and great grandparents (also born in 1800s) were lucky to survive childhood and make it to their 50s, much less past that.
The note I was responding to claimed that Europeans had the germ theory and anyone who did something bad for their health was willfully ignorant and stupid.
It's also a cultural thing. Just the other day I was minding my own business on a relatively central street in the European Eastern capital where I live and all of a sudden I see a Roma lady, in her early 50s, reasonably well dressed, coming towards the lightning pole that was on my left side, taking her underwear down and urinating, literally in the middle of the sidewalk, in plain daylight. Before that she had been talking with some acquaintances of her just outside a court-yard, I presume that if she really wanted to she could have asked those people if she could use their toilet.
Public defecation, though, I have no hesitation about shaming someone for unless it's some sort of emergency.
In the UK it is, but my bladder/prostate totally gives up, and in that situation it's a choice between a public place or my trousers.
It wasn't that long ago that Chicago had to raise all their buildings a couple meters, to help keep feces away from the people. And in the next generation had to reverse the course of a river, to keep feces away from their drinking water.
So it's certainly not that ingrained, or not since so long ago, or, as most things in life, easier said than done.
It seems obvious because it is obvious and has been, in India and practically everywhere else, for centuries.
What the parent described is probably due to other external factors (e.g. lack of choice, land restrictions or other things) that constrain behavior.
Edit for further clarity: it's unlikely these people are all "too stupid" to realize what they're doing is not healthy, but that there are other factors making it a (perceived) better choice than others (which may be perceived as worse).