“A dog's mouth is cleaner than a human's.”

The common belief that a dog's mouth is inherently cleaner than a human's is a persistent misconception, likely rooted in observations of dogs instinctively licking their own injuries or those of their companions. This natural behavior, sometimes followed by apparent healing, may have led people to mistakenly conclude that canine saliva possesses unique antiseptic qualities, thus implying a superior level of oral hygiene. Ancient cultures, such as the Greeks and Romans, even incorporated dog saliva into some healing practices, further cementing this idea in historical lore.

However, scientific evidence tells a different story. Both human and dog mouths are home to a diverse ecosystem of millions of bacteria, though the specific species present do vary significantly. For instance, while humans have around 615 types of oral bacteria, dogs have approximately 600, with only about 15% being the same species. While dog saliva does contain some compounds with antibacterial properties, such as lysozyme and defensins, these are not universally effective and certainly do not render the mouth sterile. Dogs frequently investigate their surroundings with their mouths, exposing them to a wide range of environmental contaminants and potential pathogens from sources like dirt, garbage, and other animal waste.

The widespread acceptance of this myth often stems from a combination of affection for pets and a lack of immediate, visible negative consequences from casual contact like a dog lick. People might not experience illness after a dog lick, reinforcing the "cleaner" notion. Yet, the truth is that many bacteria and pathogens commonly found in a dog's mouth or acquired from their environment, such as *Capnocytophaga canimorsus*, *Pasteurella*, *Salmonella*, and *E. coli*, can be readily transmitted to humans. This transmission poses a significant risk of infection, particularly through bites, scratches, or contact with existing cuts and open wounds (Review), and can lead to serious health issues, especially for individuals with weakened immune systems.