(A) Edward Jenner’s experiment paves the way for the first human vaccine. James Gillray’s 1802 caricature of Jenner vaccinating patients (right). (B) Ignaz Semmelweis’s recognition that handwashing with chlorinated lime water could dramatically reduce the incidence of puerperal fever in maternity wards. Puerperal fever monthly mortality rates for the First Clinic at Vienna Maternity Institution 1841–1849 (right). (C) Robert Koch’s use of new tissue staining techniques to demonstrate that tuberculosis was caused by a novel bacterial pathogen. (D) Wilhelm Roentgen’s grasping of the significance of cathode rays on a paper screen in a dark room. (E) Theodor Boveri’s studies established the chromosomal basis for inheritance and first suggested that disordered chromosomes might cause diseases such as cancer. Image credits: Edward Jenner by James Northcote, © National Portrait Gallery, London, England (A, left); Wikipedia.org (A, right; B, right; and E, left); National of Library of Medicine (B, left; C, left; D, left and right); Science Prof Online Science Image Library (C, right); Boveri, T. “Zur Frage der Entstehung maligner Tumoren” Jena, Gustav Fischer, 1914 (E, right).