OF MEN AND MICROBES: Understanding the human ecosystem. “The average human body is made up of trillions of cells. The average human body also houses about 10 times that number of bacterial cells. Scientists have been curious about our bacterial cohabitants since 1683, when Antonie van Leeuwenhoek, using a microscope he had built himself, examined his own dental plaque only to discover “little living animacules, very prettily a-moving.” But it has been only within the past few decades that scientists have begun to understand just how many varieties of bacteria live in or on our bodies. And now they increasingly suspect that many diseases are caused not by individual bacteria, but by the delicate interplay between multiple bacterial species and the human host.”