Understanding the relationship between the minute organisms revealed by the microscope and fermentation, putrefaction, and infectious diseases was achieved in the nineteenth century, primarily through the work of Louis Pasteur, Robert Koch, and their disciples. Studies of crystal structure, stereoisomerism, and molecular dissymmetry seem remote from medical microbiology, but this work provided the unifying thread that guided Pasteur through the labyrinth of his investigations. In contrast to Louis Pasteur, whose road to microbiology began with chemistry, Robert Koch came to bacteriology as a physician. Lacking Pasteur's flair for the dramatic, Koch's gift was for attention to detail and for developing simple, rigorous techniques that made modern microbiology possible. By the end of the nineteenth century, the techniques of microbiology were sufficiently advanced for scientists to name, with a high degree of confidence, diseases caused by specific bacteria or protozoa. But the infectious agents of some diseases refused to be isolated by conventional techniques.