The centuries-long quest to discover the critical role of germs in disease reveals as much about human reasoning—and the pitfalls of ego—as it does about microbes.

Two out of three soldiers who perished in the Civil War died of infected wounds, typhoid, and other infectious diseases. But no doctor truly understood what was happening to their patients. Twenty years later, the outcome might have been different following one of the most radical intellectual transformations in the history of the world: germ theory, the recognition that the tiniest forms of life have been humankind's greatest killers. It was a discovery centuries in the making that transformed modern life and public health.

This revolution has a pre-history. In the late-seventeenth century, scientists and enthusiastic amateurs used the first microscopes to confirm the existence of living things invisible to the human eye. So why did it take two centuries to make the connection between microbes and disease? As Thomas Levenson reveals in this globe-spanning history, the answer has everything to do with how we see ourselves. For centuries, people in the west, believing themselves to hold God-given dominion over nature, thought too much of humanity and too little of microbes to believe they could take us down. When scientists finally made the connection by the end of the 19th-century, life-saving methods to control infections and contain outbreaks soon followed. The next big break came with the birth of the antibiotic era in the 1930s. And yet, less than a century later, the promise of the antibiotic revolution is already receding from years of overuse. Why?

In So Very Small, Thomas Levenson follows the thread of human ingenuity and hubris across centuries—along the way peering into microscopes, spelunking down sewers, traipsing across the battlefield, and more—to show how we came to understand the microbial environment and how little we understand ourselves. He traces how and why ideas are pursued, accepted, or ignored—and hence how human habits of mind can, so often, make it terribly hard to ask the right questions.