Luckily, there’s really not many pathogens that cross from poop to the airway.

Then, the sinuses and the mucous orifices produced there are part of our immune systems, as well as dogs’.

From there, the lungs have a similar level of defense, so the chances of sniffing a bunghole resulting in a disease are extremely low.

Even with direct nose-to-anus contact, there’s very low risk. The stuff that grows well in the colon and poop just aren’t evolved to set up shop at the other end, and vice versa.