It's no secret cigarettes can yellow your teeth. But tobacco smoke has another, unseen effect. It can wipe out the healthy bacteria in your mouth, leaving the field open for pathogenic bugs—like the kind that cause gum disease. So says a study in the journal Infection and Immunity. Researchers gave a complete dental cleaning to 30 volunteers, half of whom were regular smokers. Then, as bacteria moved back in, they took plaque samples and sequenced the DNA in those scrapings. And they found that nonsmokers tended to have stable bacterial communities, dominated by a few benign species. That's good, because a healthy biofilm educates your immune system—preventing unnecessary attacks and inflammation—and it keeps bad bacteria at bay. Smokers, on the other hand, had wildly transient populations, with species moving in and out—which opened up real estate for the bad bugs. Smokers also had higher levels of inflammation, which can destroy family bacteria, too. The researchers aren't sure yet why smoking has this effect. But if you're looking for a new reason to quit, how about avoiding your dentist?