Fiction. This misconception regarding the cleanliness of dog’s mouths probably originated from the thought that most diseases are species specific. This means that most communicable canine diseases, with the exception of rabies, do not generally affect humans. While it is true that dog bites are less likely to transmit a communicable disease to a human, they are still quite capable of causing an infection.
Dog’s mouths are filled with all kinds of bacteria, depending of course upon what the dog had in its mouth recently. When you consider that dogs use their mouths much like people use their hands, the bacteria counts in their mouths can be quite high. Suppose your dog has been eating fecal material, taste testing a dead animal, licking your shoe, or simply chewing on a stick that has been decomposing on the ground-your dog could have a nice selection of harmful bacteria cultivating within the mouth.
Next, imagine that after sampling the world with his tongue, you let your dog back inside only to receive salvia and bacteria smeared kisses over your hands and face. While canine kisses can be sweet, you really should consider washing your hands and face thoroughly before you make yourself that sandwich or rub your eye.