I would probably get all kinds of diseases if I sniffed every bare asshole from every mammal I meet, humans included. I’d definitely get sick if I sniffed a dozen random turds a day. But dogs can do it without any apparent issues. Not only that, but they can do it across different species of animal, too.
What dark arts are afoot within the beak of the dog?
Dogs stomach acid is a lower ph than humans (stronger acid) which helps reduce the chances of bacteria, viruses, and parasites from getting into their systems.
Dogs also do absolutely get sick from drinking contaminated water, usually specific parasites. If you have a dog that spends a lot of time in wilderness areas, there’s a lot of vaccinations you can give them to reduce the types of problems they’ll get.
You should still try your best to let them only drink clean water, but with enough precautions, dog’s bodies can handle much more contamination that human bodies can.