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?
That’s not how bacteria are transferred. They aren’t just wafting off the turd and going airborne. If you place a shit in the vicinity of a Petri dish in a sterile field, it’s not going to grow anything.