Spontaneous generation, the theory that living things can develop from nonliving things, originated in prehistoric times and held sway throughout the Middle Ages (500–1350). One of the first scientists to test this theory was Italian physician Francesco Redi (1626–1697). In 1668 Redi demonstrated that as long as meat was covered, maggots would not “form” on it. (When left uncovered, flies would land on the meat, lay eggs, and thus, produce maggots.) Despite Redi’s findings, the theory of spontaneous generation continued to influence scientists and physicians for centuries. It was ultimately discredited by the successive experiments of Dutch naturalist Antoni van Leeuwenhoek (1632–1723), French chemist and microbiologist Louis Pasteur (1822–1895), and German physician and pioneer bacteriologist Robert Koch (1843–1910), who together proved that bacteria cause infectious diseases.