Many people let their pets, such as a dog, lick their face and lips. Is this safe or are there any serious problems with this?
That completely depends on what was in your pet's mouth. If your cat can never eat a mouse because she's an indoor cat and if your dog can never raid the trash can, having them lick your face is absolutely safe.
If they do get out and have access to potentially germ-ridden chewing toys like feces, rodents, trash or carcasses (I mean the dead bird on your neighbor's lawn), they shouldn't lick people with a weak immune system. You should also not let them lick any wound (like a scratch, rash or burn).
If the dog or cat has particularly bad breath, you can assume there are tons of germs in its mouth.
For otherwise healthy people, the chance of getting an inflammation or infection from a pet licking their face is relatively low but not zero. The most common diseases transmitted by dogs licking their owners are diarrhea and parasites. A dog bite has a much higher chance to get infected, and the infections differ from those transmitted by licks. Outside cats might carry germs and parasites from rodents or birds in their mouths, and they can cause severe infections when scratching or biting someone (which might result in amputations or even death).
DogMD states in their article about dog salva:
Dog saliva is antibacterial. “Dog saliva does contain chemicals that are antibacterial and it’s very unlikely that saliva by itself would be a direct cause of infection,” says Harvey. “You often see dogs licking wounds and that is a cleansing action and an antibacterial action to promote the healing of a superficial wound.” Of course licking won’t cure all superficial infections in dogs, so veterinary visits are still often necessary.
Dog “kisses” may transfer bacteria to humans. Just because dog saliva has antibacterial properties does not mean that dog “kisses” are clean and humans should let their guard down. Dr. Edward R. Eisner, the first veterinarian to become a board-certified specialist in Veterinary Dentistry in Colorado, notes that it’s possible for bacteria to be transferred from pets to humans. One study published in Oral Biology in 2012 found that there can be a transmission of periodontopathic species of bacteria between dogs and their owners.
About 10 to 15 percent of dog bites become infected, as do up to half of cat bites. Sometimes the consequences are deadly: In one study, 26 percent of people with confirmed C. canimorsus infections died.
In a puppy’s mouth, C. canimorsus is no big deal. At least a quarter of all dogs and many cats carry them. Humans normally don’t
That's what makes this bacterium so dangerous. It's no more aggressive than other bacteria, but humans usually don't have antibodies against it. It can reproduce unhindered by the immune system and cause a septic shock.
The same article later states:
As for dogs and their full-face slobber-fests, they’re usually not harmful — as long as your immune system is strong and you don’t have any wounds on your face or mouth that would let bacteria into your bloodstream.
If you want to allow your pet to lick you without risk of infection, simply guide them to lick your chin, jaw and cheeks and avoid any wound as well as your mouth, nose and eyes. That lowers the chances of infections because the germs don't come into direct contact with your mucous membranes.