I would assume that most people know that a dog's mouth isn't very clean, considering where its tongue is most of the time. Still, my friend argues that if you were to injure yourself and not have clean water or first aid nearby but happen to have a dog around, you should have the dog lick your wound. Is this really recommended?

-- Alex in Seattle

What luck. This question arrived right before our annual checkup for the elves' worm farm. It's always fun watching the vet try to wrangle the little wigglers. So, while she was trying to get a blood-pressure cuff on them without major smash injuries, we ran your question by her. I don't think her scream scared the worms too badly, but it's really hard to tell when they're startled.

Once she calmed down and swore she'd never, ever answer this question again, she filled us in. Of course, the origin of your friend's question is the eternal, universal, never-will-die belief that a dog's mouth is cleaner than a human's. This is probably wrong, but it depends a lot on what the dog just ate (or licked) and the oral hygiene of the human in question. Bacterial loads in spit vary according to what's just been in an animal's mouth. Anyway, the amount or diversity of types of bacteria is irrelevant to "cleanliness." Lucky for us, bacteria and other bugs are very species specific. Most of what can live in the dog environment isn't going to last in our landscape. Science guys agree that we're protected by this cross-species barrier. The same would hold true for a man-bites-dog scenario.

We did a little digging and came up with at least a plausible origin for the dog-mouth urban myth. It was based on early medical studies of infection rates from dog bites versus human bites. Conclusions were fairly consistent: human bites caused more infections than dog bites. Therefore, they concluded, human bites were more dangerous to humans than dog bites. This got twisted into "dogs have cleaner mouths," and since the science guys said it, it must be true. A little more research indicated that the original data might have reflected the fact that people tended to seek early medical attention for a dog bite and ignore a human bite. Bad idea. Blowing off a same-species bite, especially on the hand (very dirty environment), is just asking for trouble.

But, back to your friend's dog-spit-as-Band-Aid question. The vet was trying to get the worms to hold still under the stethoscope, but she managed to explain that there's no magical healing substance in dog drool. She speculates that this wrongheadedness, believed by a surprising number of folks, comes from the fact that dogs lick their wounds persistently. Why? Same reason they use their tongues for toilet paper: to keep the place clean -- to remove dead tissue and get out dirt. A clean wound will heal quicker.

Of course, we'll now undo everything we just said, which is why this controversy will go on forever. Dogs can transmit rabies, staph, pseudomonas, strep, and a number of other organisms dangerous to humans. If you're immune-compromised or have some existing medical condition, you might not be able to fight it off. So, even though the odds are low that you'll get sick from a session of licky-face with Rover, caution is advised. Especially with kids under six and oldsters. Actually, based on statistics, you're more than twice as likely to become sick from a cat bite, since cats' needle-like teeth make deep puncture wounds that are harder to clean, and again the "ignore it, it's just a cat bite" idea. Oh, yeah, and the worms are just fine, thank you.