We all want people to show us affection. And since dogs can't like your Tweets, they can lick your face. However, that might not be the best thing. For example, their mouths aren't cleaner than humans'. "The bacteria counts tend to be similar but the types of bacteria are very different," Kelly Reynolds, associate professor of environmental health at the University of Arizona, said online. The normal bacterial flora in mouths of humans are very different.
On the other hand, humans have dental hygiene to help them out with fighting rogue bacteria. Having no opposable thumb to hold the toothbrush with, doggie had to evolve ways to deal with them. On the other hand, that's still not enough. “That’s why it’s so common for older dogs to have gum disease, tooth decay, and foul breath,” Reynolds says, noting that dogs also pick up bacteria from trash and stuff. "Overall, from a health perspective, dogs' mouths are not cleaner," Reynolds says.
So while moth bacteria in Trooper's mouth is harmless, some isn't. "Dogs can carry a number of zoonotic pathogens, or organisms spread from animals to humans that cause disease," Reynolds said. Salmonella, Pasteurella, Campylobacter, Cryptosporidium, Leptospira, Giardia, ringworm, and hookworm are all joys you can contract via smooching the pooch. "Some of these are common causes of diarrheal illness outbreaks in the US," Reynolds said, noting that an outbreak of Campylobacter infections in puppies in 2017 spread to 67 people, hospitalizing 17.
Meanwhile, you might not even know that your dog is the proverbial Typhoid Marry of your house. "These can make your dog sick (usually with diarrhea), but oftentimes the dog will carry them asymptomatically," Reynolds said. "It [chances of getting sick] really depends on your individual health and the dog’s health," Reynolds said, adding that people can also carry some of these bacteria (like Campylobacter) asymptomatically.
But that only holds true for people with strong immune systems. On the other hand, the elderly, pregnant women, people with HIV/AIDs, chemo patients, transplant patients and so on should not let dogs lick their faces and should always wash their hands after petting one.
And you can't really say that you'll grow an immunity. "Humans can't even gain immunity from some of the pathogens dogs carry, like the parasites for example, so they can just repeatedly infect you," she said. "It's best to avoid them if you can." And if you can't, wash the affected areas with water and soap. "The pathogens really enter your body through the mucus membranes on your face — so the mouth, nose, and eyes," Reynolds said. "It doesn't matter if it's your dog or someone else's that licked you, you should still wash your face and hands after," Reynolds said.