It's a common myth that a dog's mouth is actually cleaner than a human's, but it is, unfortunately, just a myth (depending on the company of humans you keep, perhaps). It turns out that a typical dog's mouth and tongue are hosts to a wide range of bacteria.
We've covered Dr. Kaplan before on this topic a few months ago, but the interview in the Times is different and worth reading.
The New York Times recently set out to answer this question of cleanliness and as you would logically expect, a dog's mouth has the potential to host a whole vibrant party of bacteria and viruses. That can even include salmonella and E. coli, Dr. Leni K. Kaplan told the paper, in addition to whatever germs they acquired while sticking their noses in fun and interesting places.
Common parasites that come from another animal's stool -- as dogs have an eager attitude towards nosy exploration -- can also catch a ride. Albeit snuggling with your dog would be a pretty rare way to catch such an infection.
It all might sound like a horror show if you just let your beagle get to first base, but the good news is that anyone with a healthy immune system doesn't really have to worry very much. Your dog's saliva is a haven for microscopic critters but it's pretty unlikely that you'd get sick that way. Just make sure your pets get their shots and you wash your hands regularly. Now that I know the old myth about a dog's cleanliness isn't true, though, I think I'll be a little less friendly with Fido.
Read the full explanation in paper of record to learn more about your dog's kisses.