No Stupid Questions

Not necessarily, it is likely that a tourist is just not used to these specific pathogens. While the people living there are used to them. So their immune system isn't better per se just more adapted to the environment.

Don't tell anyone, but we third worlders convert bacteria into energy, neutralising them. That's how we survive restaurant potato-mayo salad, and street hot dogs!*

OK, I'm joking with the above. Serious now: if there's any quantitative difference on the immunity system between people living in poorer conditions, related to food poisoning and similar, I'd expect it to be a smaller component. Instead what I expect the most is a qualitative difference, between people living in different areas: you're more used to the strains of bacteria around your home, than the ones elsewhere, so when you're travelling you have a higher chance to get some stupid food poisoning.

If my reasoning is correct you should see something similar happening with travellers in general, even if they stick to places with a similar economic status as their homeland.

*or street hot dogs with potato-mayo salad. Yes, they're a thing in my city - that's why we call those street hot dogs something loosely translatable as "big rotten". (I once got food poisoning from one of those. It was not fun.)

I live in a third world country. What you don't see in those YouTube videos is how common it is for the locals to contracts sanitary-related diseases such as typhoid fever, hepatitis A, ascaris worms, diarrhea, etc. A large proportion of people I know (including myself) have contacted typhoid fever at some point in their live. Those street food resistance is earned by getting sick a lot when they're young.

There was an episode of vsauce or veritasium or cgpgrey several years ago that kind of talked about this a little bit. Basically, inside and outside of us, alongside covering all of our things and everything we touch and are around and other people, are all part of an extended network of poop particles and bacteria.

You get sick when new things get past your exterior & interior poop network of bacterial defenses. Same for anyone, anywhere. It's all just how much, how fast, and how far and how new, the new bacteria and viruses get. If it gets too far, too fast, we might die.

But, people are disgusting and COVERED IN POOP BACTERIA AND VIRUSES and so we're all fairly familiar with everything, and nothing is too different.

COVID-19, for example, was very different and spread in large amounts very quickly, I believe it's why it's called a "novel" virus. It was different enough that it just waltzed past all our defenses and killed millions of people. And then, it mutated enough, and quickly enough, that when it came back to us with the new form, our immune systems were like "damn this one virus came in here and caused a ton of damage, but for some reason we don't know exactly what it looks like. Are you that virus or it's relative?" And the mutation was like, "uhhh, no?" And the security guard/immune system was all, "okay, come on through." And it would get us sick again.

Biology is weird and epidemiology is incredibly difficult when half the population is fucking homeschooled and thinks horse dewormer helps this type of thing or that it's fake or something.

There is no such thing, generally speaking and outside of particular medical conditions, as a "strong" or "weak" immune system.

There are two different factors being smooshed together.

One factor is general health and healthcare. A healthier individual -- no dietary deficiencies, well-rested, medical conditions treated, etc.. will generally just fare better when exposed to a disease. People in richer countries typically have better health.

Take cholera as a classic example. To a person in good health with access to water, cholera is a bad time, but unlikely to kill you or do permanent harm. To a person who already had nutrient deficiencies without access to abundant, clean water, it's a potential death sentence. This doesn't mean someone from a rich country had a "stronger" immune system. That person was just able to refill their personal health bucket faster than it drained thanks to those resources available to them.

In some cases, this is literally true. For example, being underweight is a worse comorbidity than being overweight by a similar amount, all else being equal.

The other factor is the immune response. Immune response is about exposure and recovery. You gain immunity to the pathogens you are exposed to as part of the recovery process. There's very little correlation between having an immune response to one disease and being better at fighting off another, different disease (though with quite similar things like strains of the flu, there is benefit). There's no such thing as "exercise" for your immune system. There is no evidence of overall strengthening caused by more exposure and recovery. Occasional studies come out that try to make this claim, but nothing very convincing has ever happened. That immunity response, in some cases, can diminish over time without re-exposure. There is precisely one medically sound and safe way to promote immunity: vaccination to relevant pathogens.

But if we're talking about "Don't drink tap water in Mexico" kinds of situations, it may APPEAR that way.

The reason locals don't get sick from the things that hurt tourists is because... they do. They did. It already happened, you just didn't see it. If you take someone from a very rich, healthy place and someone from a poor, unhealthy place and locate them both to a foreign environment with background pathogens to which they are not exposed, both are likely to get sick.

It's usually the water that makes people sick. When people from third world countries move to first world countries and live there for a couple of years, they will get sick too when they revisit their home country.

When I travel abroad I try to drink bottled water from my home country for the first three days, after that I I drink the native stuff. A frequent traveler taught me this trick and it seems (yes I know anecdotal) to work.

Really not an expert on this stuff but I imagine there are just different bacteria in the water and that is what people are reacting to.

Last time I was at the family doctor, I mentioned that my usual body temperature when not feeling sick clocks in well below 37°C and whether that means 37 would represent a low-grade fever in my case?

She replied that 37 is an average based on observations from like a century ago when it was common for the average person to be carrying around some minor infection, and so in 1st world countries where that's no longer the case, temperatures have gone down.

It may be, then, that in the developing world, people tend to be at least slightly sick from all the pathogens around them but think of it as just everyday life and don't pay it any heed? But I honestly don't know.

Maybe in some cases for some things but overall I would say no In my experience. I've been to India 5+ times and spent a lot of time with locals. The healthcare situation for many in general is really bad. As my friend said to me, "If we get something serious there is no hospital for us, it's direct to graveyard." They get sick, they suffer, they are not superhuman immune-wise at all. The better I got to know one particular group the more I discovered that most if not all of them had an untreated medical situation that they just lived with "I have some problems in my body." This was 10 and longer years ago so perhaps things are a bit better for some in some areas now.

they're exposed to more bacteria growing up - it stresses their immune systems, promoting immunity. there was a study done years ago comparing the immune systems of children raised on farms (around livestock) and children raised in the city, and there was a distinct difference between the two.

The survivors might.