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My siblings and I went abroad to a country that doesn't have drinkable tap water, but we did not know this at the time as the people who lived there used to drink it all the time with no issues.

One day, we all decided to drink a glass of water from the tap and we all ended up in a hospital and were really ill (I'm unsure of what we had as it was a while ago). We were told it was due to the tap water and were advised to only drink bottled water in the future as the tap water is unsafe.

My question is, why did the tap water only affect us but was fine to drink for the residents there? Is it maybe because our bodies are used to clean tap water?

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    $\begingroup$ The locals develop immunity to whatever is in the water, so they can drink it with no (or relatively few) problems. Since you didn't have this pre-existing immunity, you got sick. $\endgroup$ – MattDMo Oct 15 at 17:39
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    $\begingroup$ So does this mean that certain pathogens in the water could in fact kill me but not others who are immune to them? Also does this mean that they did in fact get ill but a long time ago and then became immune after that? Also does this mean that I could be immune as I had the water and healed but with the help of some fluids that were injected into me. If certain pathogens in water kill humans how could someone become immune to it if they die when they have it? Sorry for the thousands of questions I just find it hard to understand. Thank you for your comment :) $\endgroup$ – James Oct 15 at 18:16
  • $\begingroup$ Just to add to the other answer, people in those countries do still get sick on occasion and you don’t build up immunity to all possible pathogens. Getting sick now and then is just fact of life. I spent 5 years in SE Asia and locals would get the runs just as well. They just tend to blame it on other things like ‘spicy food’. $\endgroup$ – Sebastiaan van den Broek Oct 16 at 19:58
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    $\begingroup$ @James: Many different things at work, here. You can become immune to pathogens. You can develop a tolerance (which isn't the same thing). Or you can be smart enough to filter and / or boil the water before drinking it... $\endgroup$ – DevSolar Oct 16 at 22:21
  • $\begingroup$ @AndrewT. Singaporeans had a higher chance compared to whom? $\endgroup$ – Azor Ahai -- he him Oct 18 at 18:36
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You can expect that someone drinking from a water source regularly will develop immunity to the pathogens in that water source by being repeatedly exposed to them.

However, it's not quite right to say that poor quality drinking water impacts mostly tourists. It mostly impacts the people drinking that water, especially children. Immunity comes at a cost. Some example statistics from Wikipedia:

Between 2000 and 2003, 769,000 children under five years old in sub-Saharan Africa died each year from diarrheal diseases

In South Asia, 683,000 children under five years old died each year from diarrheal disease from 2000 to 2003. During the same period, in developed countries, 700 children under five years old died from diarrheal disease.

And from WHO:

Contaminated water can transmit diseases such diarrhoea, cholera, dysentery, typhoid, and polio. Contaminated drinking water is estimated to cause 485 000 diarrhoeal deaths each year.

These statistics conflict a bit, and I won't go too far into the reasons why or try to identify the correct number. I think they are all sufficient to identify this as a substantial issue in the world.

Responses to disease vary between individuals for all sorts of reasons, including genetics, overall health at the time (for example, someone well-nourished versus poorly nourished), and access to medical care and treatment. Some you could also add up as effectively "luck". In a hypothetical scenario, you could expose 100 people to a pathogen, and of those 100, many may get very sick and a few die. The survivors may then have future immunity to the same pathogen, but it would be wrong to say the population as a whole is not affected by it, you're just looking at the people who survived.

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    $\begingroup$ Assuming that contaminated drinking water is the cause of ~25% diarrheal deaths (the majority stemming from contaminated food perhaps), the statistics need not be in conflict. $\endgroup$ – David Oct 16 at 17:00
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    $\begingroup$ Looked at another way: Later repeated exposures to some source of pathogens are less likely to affect you than early exposures, since you’re more likely to be immune. So later exposures will bring your lifetime average risk-per-exposure gradually down (and your “recent average” more so), but will continue to add to your lifetime total risk. So tourists have significantly higher marginal risk per-exposure than local adults; moderately higher liftime risk-per-exposure; and much lower lifetime total risk. $\endgroup$ – PLL Oct 16 at 17:56
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    $\begingroup$ "will develop immunity to the pathogens in that water source by being repeatedly exposed to them." Either that, or they stay ill almost constantly and/or eventually die. Death by unclean water is still a common problem in third-world countries. $\endgroup$ – Mast Oct 17 at 14:09

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