Believe it or not, the idea that a dog's mouth is sterile actually has its roots in medical literature. It has long been noted by physicians that human bites are more likely to become infected than those of other mammals, including dogs. Once statistics to that effect were published in journals and began to be repeated by medical professionals, folk wisdom took off from there.

Bite Wounds vs. Closed-Fist Injuries

Lately, however, the accuracy of those statistics has come under attack, with critics objecting that some of the human "bites" compared to animal bites in earlier studies weren't really bites at all. A 1988 review published in the Annals of Emergency Medicine found the following:

Recent study of human bites has shown that the early literature depicting all human bites as having an extraordinarily high infection and complication rate was biased by its emphasis on human bites of the hand that presented late with infection already present. These bites, the so-called closed-fist injuries (CFI), do indeed have a poor prognosis, but it may be as much due to their location and initial neglect as to the source of the injury. Human bites elsewhere do not seem to have any higher risk than animal bites, which have an infection rate of about 10%. ( Source)

And a 1995 review in the Journal of the American Academy of Dermatology concurred:

Human bite wounds have long had a bad reputation for severe infection and frequent complication. However, recent data demonstrate that human bites occurring anywhere other than the hand present no more of a risk for infection than any other type of mammalian bite. ( Source)

Although the issue remains scientifically controversial, the revisionists have a very good point. Until recently, the statistics on human bite wounds didn't differentiate between what we would ordinarily consider a bite and so-called closed-fist injuries — the type of hand wound suffered by a human being who slugs another human being in the mouth.

By their very nature, such wounds are deeper and more serious than bites passively sustained, and thus more likely to result in complications. Their inclusion in general bite-wound statistics, some researchers now argue, skewed past pathological comparisons of human bites with animal bites.