In the 1600s, nobody knew. Doctors believed that illness came from bad air or unbalanced body fluids. Naturalists thought insects spontaneously generated from mud and rotting meat. The idea that invisible living creatures swarmed inside every drop of water, every speck of soil, and every corner of your body would have sounded like madness. Then a Dutch fabric merchant with no university education built a microscope so powerful that it revealed a world nobody had ever seen.