The study of the human microbiome—the booming and much-hyped quest to understand the microbes that share our bodies—began in the mouth. Specifically, it began with dental plaque.

In 1683, Antony van Leeuwenhoek, the first human ever to see bacteria, became the first human ever to see his own bacteria. Untrained as a scholar but insatiably curious, he removed some of the thick plaque at the bottom of his teeth and examined it with his own hand-crafted microscopes. He saw multitudes of living things, “very prettily a-moving”, from spheres that spun like a top to rods that darted through water like fish. Enthralled, he soon started collecting plaque from the local citizenry and finding similar microbes within.

Mouth microbes were largely ignored for