Scientists developed the modern approach to understanding and controlling epidemic diseases during the last quarter of the nineteenth century. In 1862 French scientist Louis Pasteur (1822-1895) showed that airborne bacteria were the cause of fermentation, thus giving rise to the "germ theory," which replaced an older theory that attributed diseases to environmental causes. In 1876 German scientist Robert Koch (1843-1910)—who was studying anthrax, a disease of sheep and cattle—demonstrated that specific diseases were caused by specific pathogens (the agents, such as bacteria or viruses, that cause disease), and in 1879 Pasteur found that he could use the bacilli (rod-shaped bacteria) that caused various diseases to vaccinate people against them, generalizing from the discovery of the smallpox vaccine by British physician Edward Jenner (1749-1823) in the late eighteenth century. During the 1880s and 1890s scientists identified the pathogens responsible for many diseases, including cholera, diphtheria...