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?
Obviously there are some airborne diseases that could end up in poop in specific circumstances. But in general you don’t get sick from just smelling poop.
Smell my dogs after they rolled on some cow poop… I assure you will get sick as in vomit… (Obviously /s)