“The vast majority of microbes that live in and on our bodies do not put our health at risk, but many can cause prob- lems if their populations grow out of control. So the immune system keeps numbers in check, culling resident bac- teria here and there.

A few microbial species have found ways to sabotage the immune system and skew the balance of power in their favor. Take Porphyromonas gingivalis, a mouth- dwelling bacterium that has long been the prime suspect behind gum disease. Even in small numbers, P. gingivalis can stop white blood cells from producing certain chemicals that kill bacteria. With- out these chemicals to restrict their growth, all the bacterial populations in the mouth—including those that had been contributing to a healthy ecosys- tem—grow explosively, causing tissue damage known as gingivitis.”

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