Your dog may drag its butt across the couch or floor to un-stick a piece of fecal matter, that you then put you hand on and transfer to your mouth. People with animals in the house in no way believe this is possible, their pet is as clean as a human. People attribute human qualities to their pets, so THEIR pets don't do those kind of things.
Here is an interesting article about how dangerous keeping a pet inside the house can be.
http://www.livescience.com/48717-bacterial-infection-cat-scratch.html?adbid=10152375869301761&adbpl=fb&adbpr=30478646760&cmpid=514627_20141112_35428497