That’s just not right - about the human body having good defences and humans being fine after drinking dubious water. Drinking dirty water - even if it looks clean - still kills millions of people in the world today, and I personally got amoebic dysentery (and was horribly ill for weeks) due to assuming that the water in my student flat in Berlin would be potable.
I doubt that the French will ever tolerate Americans.
mmm
To put some numbers to that statement-
Children under five years old are at greatest risk.
That depends on how you are measuring. Surface area is Lake Superior.
My wife and I have been fortunate enough to be able to travel to the Caribbean every few years.
Usually travel to Jamaica and I have heard their water is one of the safest of the islands. And despite the signs in the resort room sink saying not to drink the water I never have gotten sick from just wetting/rinsing my toothbrush. Plus, the water line in the icemaker down the hall just has a taste/sediment filter.
Did a trip to the Dominican Republic and just wetted/rinsed my toothbrush with the tap water. About a week after I got home I had severe stomach pains. One of the first things the emergency room asked is if I traveled out or the country, second question was where? Some antibiotics cleared me up in no time. Not sure if the water was the issue or not.
One thing I’ve always wondered is if the actual plumbing is something to be worried about. If the locals don’t use the tap water for drinking, then there may be less concern about having the pipes and such be made from materials which are safe to ingest. Perhaps things like rust in the line, lead soldering, etc. may be a factor. Is that something to be concerned about?
Jasmine just can’t win.
That’s why, despite the high cost of living and the over population of this region, I’m staying right here. I live adjacent to the largest concentrated fresh water supply in the world. If we’re destined to dry up and die of thirst, we have a whole ton of drying up to do before that will ever happen here. Of course, the desperate hordes dying of thirst might be a problem.
Depends where in Europe. I have spent 2 semesters in Russia and always boiled or otherwise purified water there. In St. Petersburg, the issue was giardia - I think I was the only person in our group of 40-ish who didn’t get sick, and locals also boiled their water or drank bottled water. In Novosibirsk, it was chemical runoff from the cotton fields of Central Asia (trivia note: the large Siberian rivers all run from south to north!).
In fairness, I listed the precautions a sensible traveller takes. And I was talking about American cities, having misremembered the original post. Presumably one vacations in a place for less time as a traveller than a student. But certainly children have to be more careful and are often more susceptible to pathogens than adults. If you are travelling in a place where basic medical care to keep people hydrated is hard to access - one must be much more careful.
Does anyone take iodine tablets or filtration straws when travelling to other cities? Can’t say I have, except for camping.

There are unlucky people who seem to get sick easily from any change of food or water, though, so maybe your teacher was one of these, or knew someone who was.
My husband often has problems, so the first few days visiting family in the states we try to get San Pellegrino, which he’s used to, until his system has adjusted. Plus our local water filtration company does not use chlorine, so the chlorine smell from tap water in other localities is quite off-putting.
The water in Malta is safe to drink. It just doesn’t taste good. When we visited, the hotel provided bottled water for all their guests.
The taste of the tap water is slightly salty combined with the chlorine disinfection taste. In conclusion not great which is why over half of the island’s population and most tourists drink bottled water.

Jasmine just can’t win.
Annoyeth me not! LOL
Rusty water won’t hurt you, although it tastes awful and looks brownish, so it will be extremely noticeable. Lead will, but if you’re on vacation it’s quite unlikely that drinking the water for a few days or weeks will give you enough lead exposure to worry seriously about. Don’t raise your kid on it.