Water is nature's fruit juice, so it should be totally pure and safe, right? Wrong. Wrong, wrong, wrong. Untreated water is a breeding ground for organisms like salmonella and E. coli which are typically found in – try not to gag – fecal matter. Most countries have adequate water treatment protocols in place, but plenty of others don't. Ever heard of Montezuma's Revenge? It's alive and well in many developing countries that travelers like to frequent. Even if you avoid drinking straight tap water, you're still at risk when you brush your teeth or eat fruits or veggies washed in said H2O. The fact is, if you're used to clean, treated water and jet-set to a locale that isn't as vigilant you're at a pretty high risk for some uncomfortable symptoms [source: WebMD].
Interestingly, well water still exists in developed countries around the world, and poses the same threats [source: NIH]. So here's a tip: if you visit the home of someone who insists on using a well, opt for a bottle of soda, instead of homemade iced tea or coffee. They're used to the microbes, but your intestines are not ideally suited for such an assault!